I also struggled with the same decision about a month ago. I talked to a lot of people, and the 435 "popping out of gear" came up a lot. However, that was from "transmission builders". All the Jeep guys I talked to with the 435 said it hadn't been a problem. Didn't talk to anybody that had a bad thing to say about the T-18. The 435 has an aluminum shift cover, while the T-18 case is all cast. That and the different first gear ratios seemed to be the only substantial differences between the two. I decided on the T-18 because it was a little cheaper. I used new pilot bushing, and the AA adapter to mate it to my Dana 300. It was a pretty easy swap (replaced T-4), especially when you consider my previous mechanical experience consisted of changing oil. I don't think you'll be disappointed whichever way you go; for me, nothing really jumped out and said get this one, or get that one; therefore I went the less expensive route.
Good luck,
Highlander
82 CJ-7, 258, T-18, Dana 300 & not much else (yet)
82 CJ-7, 258, T-18, Dana 300, the rest stock so far