It’s entirely up to you. I’d go T28 while everything’s dismantled, it’s less of a hassle than having to rip everything apart afterward. Like they say, do it once, do it right. Go with the setup that you’d be the most pleased with. You’re on the right track so far…the T28 will give you slightly better power throughout the range (depending on the spec of T28 you go with anyway). Spooling wise, there isn’t much difference from T25/28, I find the 28 spools just a little later than the T25, but the duration of boost is slightly longer with a T28, not much, but enough. If you’re keen on the T28 setup, I say go for it. Make sure that if you go with a GT-series turbo of any sort at a higher boost level, that you start making fuel provisions so as to not lean out etc, you know the whole drill…make power, but make sure the supporting mods are there.
I’m assuming the T25 is the OEM turbo. If it is, you should be able to grab a seal from Nissan (there may be a little bit of a wait, shouldn’t be major). When you go to Nissan, make sure to bring them the exact part # for the seal. If not, I think your best bet is to call Garrett and ask them. You could also bring it to a local shop to get rebuilt, since the seals are gone, it’s probably wise to have it rebuilt just to avoid any future problems.
My opinion is that you should stick with the T28 setup though. But like I said, it’s your choice. Hope this helps!
Mark