IMHO, Iphone tends to die faster than ipod touch:
1) Magnetometer inside easily spoilt others parts
2) Using 3g online, Iphone will definitely generate more heat
3) Built in GPS inside phone isn't a good things, my recent phone just spoilt because i use the GPS "intensively"

but this is give and take la...depends on your budget or usage, God don't create perfect thing, so do human.