Some 15x4 wheels will protect the sidewall better.
Seriously, either will work well.
The 8" will cause the tire to 'balloon out' more and will protect the rim, but will wallow more in turns at speed.
The 10" will create more of a footprint and provide a bit more lateral stability.
I would just stick with your 15x8 rims, since you have them already.
HTH