80-90 roller 360 question

I have a similar situation with a 91 360 block & J heads. I was talking to somebody at Hughes yesterday & they said I can make more power with a flat tappet hydraulic cam instead of a roller cam. I didn't think to ask him why at the time. Can anybody tell me why? I thought the roller cam would be a good choice because it could have a faster rate of lift than a flat tappet cam. I know cam science is pretty involved, but is there something fairly simple that I'm missing here?

Also, if I decide to use a roller cam, do I need special length pushrods to go with my adjustable LA rocker arms?