Well I wouldn't consider or label Stoudemire as being "injury prone", it's not like he's a Bob Sanders, Kris Jenkins, Greg Oden type of athlete who can't stay healthy.
One thing you have to keep in mind about Dwight Howard; Yes, he may be a little younger than Amare, but he does play with a much more physical & aggressive type of game when compared to Amare down low. If I had to look into the future? Based on both players-playing style? I'd guess Howard starts to break down a lot sooner than Amare.
With all that said, it breaks my heart to say it, but yeah; you'd have to trade Amare for Howard if the opportunity presented itself.
Dwight Howard is the greatest Center the NBA has seen since the Hakeem, Ewing & Robinson era. Shaq & Howard of Shaq's prime would have been an all out war.
Dwight can put up 20 points per game, get opposing BIG's in serious foul trouble, the best defensive player in the NBA right now (KG once held that crown) & is the youngest NBA player of All-Time to record 7-8,000 rebounds.