Honestly I think you have to try a bunch of different things and see what works. I do a lot of research on nutrition and diets (I'm a freelance writer- I mostly write about food, public health, etc.).
After reading literally hundreds of books on every diet/food research under the sun- I've come to a couple of conclusions:
1. We are always looking for the holy grail diet. I don't think it exists. I think your body type, physical activity, genetic makeup, and even ethnicity can contribute to what works for you. I think in the future we will see advanced DNA scanning that provides more information on what is an optimal diet for different people.
2. Eating a lot of red meat isn't great for you, and a lot of milk isn't great for you either. I'm pretty convinced on this point. Am I saying that I am 100%, beyond the shadow of a doubt, positive that this is true for everyone? No. But I would say that the research backing up this statement is very strong. More studies (with more people and over a longer period of time- and no, not just correlative) have been done on this specific dietary block than on just about anything else. You can argue that it's actually the modern production of meat and milk that makes it so bad. That may be, I don't know of any study that compares individuals who don't eat meat or drink milk, with individuals who eat only the purest meat and drink raw milk (though that would be interesting).
As to your question on vegan or paleo- first I would point out that paleo isn't really a new concept. I don't think there's much evidence to support most claims made by the paleo community (blanket statements like NO grains are good for anyone, that humans need lots of meat, etc.) I believe (based on my own experience, interviews, and research)- that many people see lots of positive effects after transitioning to the paleo diet. I don't think this has much to do with the paleo diet itself, or the concepts behind it- but more to do with the side effects anyone would experience when switching from SAD (Standard American Diet) to a whole foods (no sugar, no grains, no processed foods) diet. If you're paleo, you're not eating a lot of garbage junk food- which is great. I do not think it has anything to do with our species needing a ton of meat or grains being poison. In fact there is quite a bit of research that says originally humans probably ate closer to a vegan diet, and populations who eat large amounts of grains have historically had fewer cardiovascular diseases and cancers- not more. However, if you go from eating pizza and spaghetti and ice cream every night, to eating nuts, lean meats, and vegetables- yes, you will probably feel better.
Vegan diets work for some people but not others. It's possible to eat junk food every single day and be vegan, or to eat only pure organic produce and be vegan. I think it's hard to know the full health effects because it is difficult to find huge populations that maintain a vegan diet from cradle to grave- which means many studies don't have a large vegan population to draw from. Vegetarianism has been better documented in studies (especially since it is mandated by certain religions- so you have large populations that are vegetarian their whole lives). There is a good amount of research that supports that a vegetarian diet makes certain diseases significantly less likely (cancer, heart disease, obesity). If you want to play devil's advocate, you could say this is because many vegetarians (who are so for religious reasons at least) also wouldn't be smokers, drug users, or drinkers. It's hard to know. Dietary research is very complicated. I would point anyone who disputes this to The China Study- which is the single most comprehensive study on nutrition ever done. I think the evidence is quite strong that meat (especially red meat) and an excess of dairy has harmful effects.
Personally, I was a vegetarian for 10 years (ages 9-19), I tried vegan for about a year (recently). I also ate a very heavy meat diet for about three years (paleo + grains) (ages 20-23). I have played with my diet a lot, eliminating different things at different times, and here's what works for me:
1. I Need Lots of Fat- I am naturally very thin- If I were an animal I'd probably be a spider monkey, long limbs, super-fast metabolism, but really hard bottom out if I don't get enough fat. I eat a LOT of nuts, avocados (at least 1 a day), and a fair amount of butter. I eat small amounts of cheese, because I love it. I eat eggs very moderately- maybe an average of 3 eggs a week. I don't eat/drink milk, heavy cream, yogurt, or any other dairy products.
2. Grains- I eat rice every single day. I also eat spelt bread. I don't really eat any other grains- they seem to make me bloat.
3. Beans, Greens (spinach, collards, etc) & Cruciferous vegetables (broccoli) are not good for me. Same problem as number 2. They also make me feel sluggish.
4. Fruit- I eat a ton of fruit. In the course of a day I may eat on average : 4 oranges, 2 bananas, 2 kiwis, 1 cup grapes, and 1 tomato. Too much sugar does not seem to be an issue for me. I LOVE FRUIT.
5. Vegetables- I eat lots of winter vegetables (sweet potatoes, potatoes, squash). I also eat lots of tomatoes, green beans, zucchini, all kinds of peppers, garlic, ginger, mushrooms, celery, leeks, carrots.
6. Seafood- I eat a lot of seafood. Mostly shellfish, but I also eat tuna and salmon. I eat heaps of shrimp, clams, oysters, and crab (when I feel fancy). I eat tuna and/or salmon probably twice a week- I eat some kind of seafood every day. I also eat a lot seaweed. I know it's odd- I played with my diet for years and years. Something about seafood just makes my body work like a machine.
7. Red Meat & Poultry- As you probably guessed I don't eat much of this. Maybe a handful of times a year. I don't like my own dietary preferences made an issue- so If I come to your house and you serve me steak and pasta, I will eat it. Very occasionally I will also get a small piece of high quality steak to make myself- this is only when I'm on my period, which makes sense. Occasionally when I have a particularly strong period I will crave red meat (Iron!)- this is pretty normal for women, for me this is the only time I crave red meat, I'd say this happens to me maybe three months out of the year. So other than my 3 time a year "lady steaks" (yep, I went there) and being a good guest, I don't really eat much from this category. I think American poultry is basically garbage- unless you're buying from a farm that you know all about or something, it's just completely pumped full of antibiotics and growth hormones.
8. Herbs & Spices- These get overlooked a lot- but herbs have major nutritional value. I strongly spice my meals, because I grew up that way. I feel good when I eat lots of spices and herbs. I don't experience heartburn or upset stomach from even large amounts of curries, paprika, garlic, tumeric, oregano, basil, coriander, etc.
9. I also eat a fair amount of fermented things- like Miso and Kimchi. The health studies are all over the map on fermented foods- it's probably too early to tell for sure how positive the effect is. I haven't seen any research that suggests they're bad for you. I say eat them if you like them, if not skip it. I just like the tangy taste.