A year or so Street Rodder mag did a test of roller vs. flat. The hydraulic roller starts to make more power than flat tappets with "big" cams. Like above 280'ish duration. Below that, there doesn't seem to be much advantage performance wise.
Another big issue is durability. Flat tappets are going south at an amazing rate. Part of it is
oil, and part of it poor quality of the available tappets. Whatever the reason, I would be inclined to use a roller just to avoid it. It's expensive, but may save a lot of money and time later on.
When picking a cam, carefully read the description in the catalog. remember that bigger cubes / strokers / forced indutcion tend to soften the cam. Pick the cam you want, then go one step up.