http://garage.grumpysperformance.co...-the-extra-cost-vs-a-flat-tappet-design.3802/
ROLLER
less friction
faster valve lift
longer effective duration
less chance of lobe failure
you should also keep in mind that a roller cam valve train with the same lift and duration as a flat tappet cam, can provide a good deal more port flow potential and resulting power, , how much more power is mostly the result of how much more port flow and volumetric efficiency you can get from the engine combo that the flat tappet cam will not be able top access because the valve opens longer per degree of rotation on the roller cam lobe and the lift and duration,the valve sees increases, roller cams have a far lower percentage of lobe wear and less friction, both tend to result in more usable horsepower.
now if your current flat tappet cam is providing most of the engines potential then the roller cam will only provide a mild improvement, but in most cases you can expect a
15hp-40 hp gain, (depending on displacement, port flow,exhaust scavenging etc,)over a similar flat tappet design with similar duration at .050 lift, but in many case the roller cams peak lift will be noticeably more.
within certain limits ,the larger the lifter diameter the faster the cam lobe acceleration ramp can be, chevy has a lifter diam. .842
ford uses a .874 diam, Chrysler .904 diam.
the roller will easily allow you to exceed the 550 hp level, I'd go that route,
yes the flat tappets initially cheaper, but youll very likely regret going that route later if you did go that route