8 1/4 vs 8 3/4???

so some of you told me for my daily driver project i should go with an 8 1/4 suregrip for my 360 dart (when it comes haha) anway ive seen a lot of people doin 8 3/4 suregrips with the same engine. what differences would i see as far as performance, mileage, ect if i went with a 8 1/4 or 8 3/4? i need sbp so is 1 easier to find in sbp that the other. thanks!