I promised myself I would stay out of this thread, but some of the comments here are forcing my hand.
First, my career has mostly been in valve train and bearings. I am one of the guys in the automotive industry who designs those little bearings that go onto roller lifters. I've been closely or loosely associated with OEM valve train design for most of my 16 year engineering career. So, I at least have a little bit of a clue what I'm talking about.
There are two main reasons why a roller cam is "better" than a flat tappet cam...
1. A roller cam and lifters will have lower overall friction than a comparable flat tappet cam.
2. A roller cam can allow higher acceleration rates, meaning it can open the valve and close the valve more quickly, than a flat tappet cam.
Regarding Item 1: rolling friction is lower than sliding friction, period. Yes, roller cams often need stiffer valve springs than flat tappet cams because of their higher accelerations. This does not necessarily mean that the roller cam produces more friction. It takes more force to open the valve, indeed. However, when the valve is closing, that same valve spring is now pushing on the cam in the "good" direction and helping it rotate. So, whatever work is done to open the valve is recovered when the valve is closing. All the while, the rolling contact is generating much less friction than a sliding tappet would generate. As a side bonus, rolling contact offers much less wear than sliding contact, so wiped-out lobes are almost a thing of the past (even with today's crappy engine oil).
Regarding Item 2: Because of the sliding contact, a flat tappet cam is limited in the shape it can have. The cam's radius of curvature must remain positive and large to keep the contact stress low, and the radius of curvature can never go negative. With rolling contact, the radius of curvature can be much smaller, and negative radius of curvature can be used. If you look at many modern automotive cams, there are portions with negative radius of curvature on the flanks of the cam - this generates high accelerations and offers more area-under-the-lift-curve than flat tappet cams can offer. This, in turn, offers more performance for the same amount of valve lift as a comparable flat-tappet cam.
For the above reasons, most modern engines run roller cams. Believe me, it would be nearly impossible for the modern GM small blocks to make 400+ hp and pass emissions tests without the benefits of a roller cam. Some manufacturers still run flat tappets - the Hyundai and Ford 4-cylinder DOCH engines come to mind - they have a cam directly acting on a flat tappet bucket lifter. The primary reason they do this in some DOHC engines is to keep the valve train (the stuff that moves, anyway) as light and small as possible. This helps the engine to reach high RPM's, provides more room in the head for the ports, and makes the cylinder head assembly smaller so it packages under the hood better. These are just the basics, there is a lot more to valve train design than what I typed here, but you should get the idea.
Having said all of this, I am still running the stock flat-tappet cam in my engine. I will probably keep running it until it either wears out or I get a big desire to run much faster than I am now. If/when the time comes to replace the cam, I will put a roller cam in there, no question, for all of the reasons I stated above. That's not to say that flat tappet cams are terrible and don't work - they do. But, my experience tells me that a properly designed roller cam will generally outperform a comparable flat tappet cam in most applications.
Take this for what you will. It's always your money, so it's your choice what to do with your engine.
Good Luck,
