Our first trip to WDW was when my first two children were 16 months and almost 3. The almost 3 age was wonderful. After that trip, I insisted that we go back when dd was about that age so that she could have that experience too. Wound up going back when she was just turning 4 and son was almost 6. Ideally I would have gone right before she turned 3, so she would be free, but for us, it was too close to our previous trip. She loved the princesses, and was pretty obsessed with all characters in general, while my son just wanted to run from thrill ride to thrill ride. He still had some interest in kiddie rides and characters, but it was definitely fading.
Now we're going to DL when the youngest is 2.5. When they see the characters at that age, it's just different. It's great when they can ride the big rides, but there's just some extra magic when they're 3. Dancing at the parades, gasping at the sights, and just being full of wonder. DD (turning 6 on the trip) now is obsessed with the fact that the characters are people in costume, and she talks about it and asks questions non-stop. This trip with her will be a very different experience from the last time.
I'm sure we'll be back again once the youngest can ride everything, but I'm so glad that we were able to accomplish my goal of having them all go around the age of 3 to have that unique experience that you just don't get when they're older. It's still a fun, amazing trip when they're older, but not quite the same experience.
As far as behavior and tantrums, my oldest on our last trip who was then almost 6 had far more tantrums than his 4 year old sister, and caused WAY more problems in line than the earlier trip when they were 1 and almost 3. The first trip when they were younger was much easier for us.
Strollers don't bother me, and I'd actually prefer a trip BEFORE they're potty trained. You can wait a couple minutes to change a diaper, once they're potty trained, you have to worry about the time when they need to go NOW.
For memories, my kids are obsessed with looking at our Disney pictures. I don't know how many memories are "real" and how many are from stories and pictures, but they know they went to Disney, and they know that they had an absolute blast!
If you can swing more than 2 trips great, but ideally, I'd say do two trips. One right before 3 when you don't have to pay for them, and they can enjoy the characters, parades, kiddie rides, etc. And, then one later when they're taller and can hit all the thrill rides. It really makes for very different experiences.