I'm hardly a guru but here's my advice:
Drop that Marriott card like its a hot potato.
Stay away from Delta, awards are a waste of time.
US is merged with American, soon will be oneworld partner. Your focus therefore could be American, or if your travel habits are for domestic travel you should aim for Avios from British Airways and use them to fly on American metal (which the US soon will become).
Tell me more of your habits or goals and Id be happy to get into more details.
It would be for domestic travel, most often to Boston or Portland (ME). We generally earn ~2500pts a month on the Marriott card. Like I said, it has worked out great because for the $65 yearly fee we automatically get 1 night at a cat 1-5 hotel, and it gives me a 10night boost to my status (just dropped from Platinum to Gold). We used points to stay in downtown Atlanta for two nights earlier this year for the Falcons/Patriots game.
Generally we use the Marriott points or free night stay for something within driving distance for a weekend type trip. Right now I'm looking at Savannah for Valentine's day for example. The flights to Boston/Portland would have us staying with family so we wouldn't need to combine flight/hotel.
So its not that I don't use my Marriott points or don't like the card, its just that I figured I should look into these airline miles type rewards and see if that might be even better. We fly out of Charleston, which has fairly shitty options (and prices) since we're a destination, not a hop. If I could get two tickets a year for free ($600-$1000), I think it would be worth changing my spending from a hotel points card to an airline points card.
edit: just checked my rewards activity, we stayed 3 nights on points this year (which was 65k pts). I still have a certificate for a cat1-5 stay available that expires in March. I'd say 2-3 nights per year for personal fun use is probably average.
There are two things you need to consider when it comes to any new card, they are: are you seeking signup bonus cards, or are you seeking everyday spend cards.
Frankly anyone on here who is in half decent financial shape should be applying for several cards every 91 days or so, capturing lucrative signup bonuses and using that to cover their travel. There is an inherent risk in new credit cards in the wrong hands, but if you are able to show the discipline that got you here today then you are likely to be able to control the risk. The phrase App-O-Rama is given to the multiple card applications, it is done in batches like this as it means that they fall off your credit report in the same batches that you apply, so it allows you more cards.
The risk is that you think of credit cards as actually credit, rather than use them for perk abuse.
For the everyday spender card, the truth is that there is no 'one perfect card' this is because cards offer category spend bonuses, some, like the DiscoverIT and Chase Freedom offer rotating 5% cashback categories (changing each quarter) whereas others, like the Chase Sapphire Preferred offer 2x for Dining and Travel expenses.
Regarding your specific case here is the analysis you need to consider:
Marriot points (like all programs) have earning rates, and burning rates - by manipulating either side of the equation you can control value. In order to create a apples-to-apples comparison of the value of this card to you, and the opportunity cost of it you need to consider the following:
Earning Multipliers (not good value)
5x at a Marriott (you should be spending zero at a Marriott unless it is a reimbursable business expense) so this has no real value to me.
2x on Airfare - this is an average earning multiplier
1x everywhere else - this is bad.
The problem with the program is that thinking of it as your everyday spend card, you need to spend $50K per year to earn 50,000 points, which in turn are worth a night in a hotel. The value of the hotel to you sounds like it should be less than $100 per night?
Therefore you are swapping $50K for $100 in rebate.
If you instead put that onto a card like the Barclaycard Arrival you would get $1100 in travel costs to spend it on, or 11 or more nights at a similar hotel.
If you look at this post, I compare spending $20K per year using better cards and a better mix, it might be useful:
Here is a post I wrote on the best card for low spenders (in my world not yours!) which shows value on 'everyday spenders' if you are interested:
http://saverocity.com/travel/best-credit-cards-low-spenders/Regarding the switch to an airline card - you should but not to earn everyday points, it should be to capture signup bonuses.