Just go by what the tyre manufacture recommends, I still don't understand the logic behind people stretching tyres, if you can't fit the rim under the guard buy wheels that fit, don't stretch tyres.
Not having a go at the OP btw, obviously a 10 will fit on a GTR but just so sick of seeing all this stretched tyre crap.
Both tyre sizes are listed as fitting a 10 for the rsr, so either is fine.