First, the cams usually cost a substantial amount more, especially if you're trying to get retrofit roller lifters for a block not originally machined for a roller cam (usually anything pre 1987 or so depending on the manufacturer).
Second, roller lifters weigh a bit more and limit RPM unless some pretty heavy-duty springs and associated valve-train gear is used.
Do a search on this topic, its been done to death-
To be honest with you I've never used a roller cam in any engine that I,ve built. Just could'nt justify the extra cost for the application on a good street rod engine. From what I read the "up's are obvious, more power due to less friction, etc. but the "downs" are equally obvious....$$$$$$$.
Now if you're really on a shoe string budget you can get a hydraulic flat tappet cam AND lifters from Summit for $69.95 But I would really have to question the quality of both. A roller cam will cost $600-700 on up from what the prices show in several publications. Its all up to you and how much money you want to spend, as Killer said do some homework.
My car came with a Crane solid roller cammed LS-6 454 big block.That cam got ruined by a lifter that broke,that setup was probably 16 years old with quite a few miles and lots of hard use.I replaced it with a solid roller Comp Cam set,but what I didnt realise at the time is that Comp doesent make their street line cams out of billet steel.I'm rebuilding my engine this year and I will be putting a custom ground billet roller cam in.A couple of things to keep in mind are dont use oil restrictors or crank scrapers,roller lifters get their oil from the oil galleries as well as from crank windage.Use top quality roller lifters,Isky is offering a no needle bearing option on their Red Zone lifters.You must also use the correct distributor gear ,the wrong one will wear out very quickly.Cam travel must also be controlled,a cam button between the cam and the timing chain cover and a thrust washer between the upper timing gear and the block should be used.The correct springs for the cam are a must,as well as top quality pushrods,screw in rocker studs are a good idea too.Theres probably more that I've forgotten,but hopefully this will give you a better picture.