Flat tappet:
Good for high rmp but noisy, if it's too agressive, it might eat a lobe.
Hydraulic flat tappet:
Pretty good rpm, quiet, if it's too agressive, it might eat a lobe.
Solid roller:
Extreemly good for high rpm, may need very stiff valve springs, the needle bearings in the rollers get beat up if the valve springs are too stiff, noisy. They also have less friction because the roller rolls across the cam lobe, compared to a flat tappet that spins/slides over the cam lobe.
Hydraulic roller:
The lifters are heavy, that's bad for high rpm. They like a stiffer spring than a hydraulic flat tappet. If the spring is too stiff, the oil in the lifter can get pumped out and that is bad at any rpm. If everything is just right, you get silent opperation, ~7,500 rpm before bad things start happening and the improved ramp rates and friction that a roller lifter offers.
The hydraulic flat tappet with a conservative cam is very reliable.
Solid flat tappets are almost as reliable, but require regular lash checking/adjustment.
Solid rollers have the potential to be as reliable as a solid flat tappet, but most solid roller lifters are run on pretty wild solid roller cams which make a vavle train that isn't so reliable.
Hydraulic rollers are known to last 200,000 miles with a stock style cam if the RPM's are kept under controll.
The hydraulics are convenient, they don't require adjustment, they are quiet and they do a good job in daily driving.
Solids have more potential. More power potential, more rpm potential, and more potential to break.
It's all a trade off. It you don't mind checking the lash occasionally, a solid can be a lot of fun. If you love high rpm, a hydraulic roller can be a drag. If you love to putt around and go to car shows, a hydraulic roller might be perfect for you.
