It's not so simple as saying "it comes down to the lobe designs for both cams". Sure, it is possible to have a flat tappet that is more aggressive and has more area under the curve, than a roller. But, there are downsides to that, as well. If you have a very aggressive flat tappet, you need more spring pressure to control it, unless the rpms are kept low. Since increasing spring pressure eventually becomes problematic with flat tappets, that can be an issue.
I once used a very aggressive solid flat tappet cam, 270/240/152 on the intake side. Had some trouble controlling the valve train with the initial setup. It started going uncontrolled around 6000 rpm. We had to change to springs with more pressure and eventually went with titanium retainers. The motor ended up peaking at 6200 rpm and pulled well to 6500 rpm. I can't remember the exact pressures, but I think we started with around 360 lbs open and ended up around 390 lbs open pressure.
I now run just over 400 lbs open pressure (150 on the seat) with another solid flat tappet combination. I also have a nitrided cam, EDM'd American made lifters, and run good oil with a ZDDP supplement. So yeah, it's doable to make a flat tappet run with a roller, up to a point, anyway. But there are some downsides unless it's pretty mild engine. So it's not so clear cut, in my opinion.
My current cam is 279/252/162 on the intake and 287/260/169 on the intake. Those are 0.020"/0.050"/0.200" numbers. Lift is 0.340" on both.
Even if I use a extremely mild solid roller lobe like Comp High Energy 288, I can get 244 @0.050 and 158 @0.200. The High Energy 300 gives you 255 @0.050 and 170 @0.200". So I can get more at 0.200" but pay a penalty in longer advertised duration. That's comparing an aggressive flat tappet to a very mild roller. Peak lift isn't everything, but of course it's higher too.
Anyway, I'm not ruling out a solid flat tappet. There's a Harold Brookshire (Berkshire?) flat tappet design with 277/244/157 degrees that I like. I think that'd make a nice intake lobe.
This will be a 471-474 inch motor if all works out well. I know the cams I pick should peak lower, but the heads I have seem to help. My old 240/246 cam peaked at 6200 rpm in a 433 inch version, for instance.
So long story short. Yes, a solid flat tappet makes sense, and so does a hydraulic roller. I don't like hydraulics and I want to be able to easily swap cams on the engine so I'm thinking solid roller. I think it'll make more power with the roller unless I get fairly crazy with the flat tappet. Of course it depends on the exact lobes used, but there you have it.
Sorry for the hijack!
paulie