I've never gone to Disney while pregnant, but I have gone with an almost 4 month old and a 7 month old, and both trips were really pretty easy, especially with the younger baby. I don't think I would do it when it's really hot, but both of my baby trips were in November so it was perfect, not too hot, not too cold. I breastfed my babies so they were very portable, there was not too much extra to tote around, but even with a bottle fed baby I don't think it would be too hard.
On the contrary, I never really felt that great during my pgs and I know I would not have had the energy to really enjoy the trip, let alone it would be frustrating to go and not be able to do the bigger rides that I enjoy. If it were me, I would take the trip with the new baby.
Whichever you decide, have a great time and congrats on the upcoming new baby!