It would help if we knew how you were gonna drive it, and the intentions, all out performer or warm street driver.
I am NOT a small block guy, but obviously each has pluses and minuses, in any engine family. The roller costs a bunch more, probably triple the flat tappet. It will probably make 25-30 more hp, simply because a roller profile can be more aggressive with more lift at almost all points on the curve.
But if this is primarily intended to be a street, fun, engine, I would lean toward the flat tappet. If you can get it broken in properly, it won't have the potential of broken roller tappets in extended street use that can happen with an aggressive roller.
So, in my opinion..... frequent drag strip visitor, occasional street use (1500mi/yr), roller. (If budget allows). Mostly street use, with a visit or two a year to a strip, 5-10,000mi a year, flat tappet.
Again, MY opinion only.