Having learned on an Aerostar 20 and flown different guys Alpha's, Tower Trainers, and my own H9 Arrow(s), I would agree that you're not gonna go wrong staying with any of the "standard" trainers. And if you're relatively young and have great eyesight, starting with a 40 size trainer makes financial sense.

But if you're a bit older, and your vision isn't quite what it used to be, the extra size of a 60 sized trainer might help in the learning process. I find that with larger airplanes the "spacial disorientation" problem seems smaller.

Another consideration that might be relevant is what you may want as a 2nd or 3rd aircraft. There are a number of ARF/kits out there (Sig 4 Stars, Top Flite kits, etc.) that are 60 sized. Every now and then when I look at my pair of .46 Magnums, I wish one of them were a .60.