Here are the reasons for running a solid roller or a hydraulic roller as well as a solid flat-tappet or hydraulic flat-tappet. The reason to run any roller cam design is to better fill the cylinder with the same duration. With the roller design you can have a more aggresive design and not worry about wiping out the lobes. The flat tappet designs can not run as aggresive a lobe design because of the lifter design. With the lifter design being about square or flat it wears the lobes.
The solid will rpm higher because it is not relying on the
oil to keep the lifter pumped up. A hydraulic lifter responds to lighter valves and retainers to get higher rpms. The hydraulic flat tappet is good for about 5500 to 6000 depending on these things where as a hydraulic roller is good for about 6200 to 7000. The same goes for the solids where the solid roller will rpm higher and be safer in doing so.
The main reason most people go with the hydraulic stuff is because they do not want to adjust the valves and the solid rollers will last about 6000 to 10,000 miles depending on brand and the spring pressure used along with how aggresive the lobe design is. This is why we use a lot of hydraulic roller because they last longer than a solid roller and they make more power than a flat-tappet. I also do not like to pull a engine apart when a lobe goes out on the flat-tappet stuff and makes a mess of the engine. Maybe this helps a little bit. Keith