i am no expert but it goes like this
-5 AKA HKS 2530 - Good top end and good response also in RB26 with build internals and head work / Stroked engine, but for normal RB26 with stock internals / street driven a bit laggy, "this laggyness is not always clearly shown in a Dyno apparently, but noticeable on the road" when compared others. lot of people with -5 street driven and loving it
-7 AKA Nissan R34 N1 - good response and good balance of power for Street driven GTR , but by no means a bragging rights turbo when it comes to numbers
-9 AKA GT-SS - best response and good power for street package, but will run out of puff in the top end, better and new technology than -7 and most recommended for street package with mostly stock internals
-10 AKA GT-RS - big power figures and not so recommended for street GTR without built engine.
I will be getting -9 end of the year, not really interested in any thing else, unless -7 comes up in the forums for really good price
Just my 2C
Also check out single turbo conversion side of the world, if your thinking of GT-RS level of turbos.....
all around, good choice we have in GTR world i think