Roller is the ultimate in performance, but costs more money. If you want to save a couple of hundred, go with a flat tappet. The flat cam prevents the aggressive ramp rates that roller is capable of, so it makes less power.
There are a lot of sportsman class racers and hot street cars running flat cams. Both hydraulic and solid.
I run a flat hydraulic and it works very well. Even in my drag car. At this point, I need to upgrade the cam to go faster, but so far I am happy with everything, so I leave well enough alone.
converting a flat tappet to a roller motor is pricey, but i think it might be worth it. the roller tappet allows you to get much higher lifts out of a given duration vs. a flat tappet. roller motors i think are a bit quieter and it can really make for a well-mannered street/strip car
a roller cam has more area under the curve as opposed to a flat at the same specs. which equates to valves opening faster and staying at higher lifts longer