well, from what i have heard, the gt30 variant is better, more power with less lag. i only know them as the 49,51 and 53mm comp inducers. the tiny little gt28 turbine is a major bottle neck. much like the older gt2540r.
on a s15, the 51mm aka -10. came on at 3800, changed to gt3071 same comp wheel, came on at 3300. and made more power.
so if you are thinking of getting one, get the gt3071. great turbo.