<laughing> And a second tummy sleeper chimes in the opposite direction. I'd ratherhave no bed at all than on a firm mattress. LOVE soft mattresses. Pillowtops & down. (A good bed won't sag, no matter how soft it is.
So now that the conflicting advice has come in...two thoughts:
1) How attached are you to a toddler bed? Regular twins come in a waaaaaay larger range, last for 10 years instead of 2, and cost about the same. Cute sheets and you're set. You can also just put the mattress on the floor, then raise it on top of the box spring when rolling off isn't a fear. (We actually went with a double for our son, because we like laying down and reading/talking together). We also got a super expensive bed for 30% of the retail price because we went to an outlet, instead of storefront. So it ended up at about 300, but would have been about a grand. Flukish, they had it, he liked it, and that was our price cap.
2) Would you be willing to take him shopping with you? Have him lay in a bunch and see what's "comfy"? It helps give them ownership, kind of like how they'll eat food they make themselves that they wouldn't touch if someone else made it, and (at least for us) helped a ton with the transition.
Anyhow... just some thoughts.