This is probably the hardest decision we've had to make and right now I'm grieving pretty hard.
I really feel for you. We were in a similar situation one and a half years ago. I wanted a third, my husband was on the fence. It was by far the hardest decision we ever had to make as a couple. If I had said we were done at two, my husband would have been perfectly happy with that. But I just always had that nagging sense that our family wasn't complete. Looking long term at our lives, I knew I would always be asking, "What if?" and looking at that empty chair at the dining room table. So, after many, many, many conversations we decided to try. I got pregnant immediately and miscarried after 6 weeks. It was terrible, and after that I knew that I truly would be heartbroken to not have the third. As hard as it was to imagine miscarrying again, we started to try again after a month and immediately got pregnant again.
Our third son was born in May and is now almost 4 months old, so I am very early on the third kid journey. For me, going from 1 to 2 was absolutely brutal. They were 24 months apart, and the first was the worst sleeper known to mankind. No, really. A nurse who specializes in pediatric sleep told me so. So we had a newborn and a toddler who was still waking up 4 or more times a night at that point.
In comparison, this time around it is easier, because my two older boys are 6 and 4. I think that really makes a difference. But you really can't predict which transition will be hardest or easiest, since it is highly variable on the kid, you, and all the other circumstances in your life.
One thing I will say for sure is that you have more perspective the more kids you have. You are more likely to not get caught in the moment and can realize that "this too shall pass." Of course, I say that, having had a meltdown earlier today about my infant's naps. But that just comes with the territory of infants and postpartum hormones.
Regarding the pregnancy, I was 36 when I had my third, and yes, it was more difficult in many respects. At one point, they thought I had an embolism, because I was having trouble breathing. After a round of tests (very expensive tests!), they just concluded that I was old (i.e. advanced maternal age) and just needed to slow it down a bit. That was discouraging. One perk, though, is you have more kids and have had vaginal births - the baby comes out super fast. I pushed for a grand total of 2 minutes. One push and he popped out.
I honestly don't know how I'd feel if my husband had been firm about only having 2. One thing that I made certain to communicate to him is that we certainly wouldn't be having the same conversation again in a year or two. In other words, I would definitively be done at 3. And this is true! You really do know when you are done, and I am finished. I had some early on mourning about the fact that I would never be pregnant again or feel a baby kick inside me again, but that was mostly the baby blues. I'm already over it in 3 months and now look forward to giving away the baby clothes. I could never say that before.