Don't take this an attack but I'd like to see some data to back that up. I've been running on/off stats for years now without issue. Ironically, my first proportional tstat that I paid 4x more for failed after 4 months and had to be replaced.
Whatever you use you should always spot check temps regularly with other devices such as a temp gun.
I will say though that if you can afford a better one then go ahead and get it. But if he's go all his snakes in seperate locations now (like they're in tanks?), I'd go ahead and drop the $30 and get them moved in the rack and upgrade the tstat later.
I agree with mason, there is not any data to back this up that I can find. Balls need a gradient between 80 and 90. Proportional thermostats are higher tech but i don't think they are less likely to fail. Either type will give that gradient if set up properly.