Well what's your rear gear ratio?
I've used both sizes.
The 245/60R15 (26.5 inch diameter) Mastercraft's I had were decent at the time I had it (I had 3.21 rears)
My only complaint is that they didn't last that long (BFG was slightly better)
I now have 255/60R15 (27 inch diameter) Cooper Cobra's with 3.55 gears and I love it. So far the verdict is out as these are still brand new, but I think these tires are comparable to BFG's but at a much better price.
If you have a 3.55 rear, I think the 255's are the way to go.