STI has redesigned their aluminum grip, they just aren't available yet. I have a couple of the first generation STI aluminum grips and haven't had any problems...yet. The knock on aluminum is that it work hardens and becomes brittle...which is why you get cracking. This is likely to occur in any aluminum grip framed gun at some point...doesn't mean it has to but it most likely will eventually.
IMO, If you can afford the extra $, get the SVI steel...you will not be disapointed.
I am a little worried about the weight of a steel grip and how that will affect the balance of the gun. I am not a big dude and a little concerned about driving a heavy gun. Any thoughts from those who have SVI steel grips?