What are the main differences in OHC and OHV engine designs, and which is faster or more powerful? Also, which is more reliable?
OHC refers to overhead camshaft and OHV refers to overhead valve, two different configurations in engine design.
The key performance differences are described briefly in this Wikipedia article
The fundamental reason for the OHC valvetrain is that it offers an increase in the engines' ability to exchange induction and exhaust gasses. (This exchange is sometimes known as 'engine breathing'.) Another performance advantage is gained as a result of the better optimised port configurations made possible with overhead camshaft designs. With no intrusive pushrods, the overhead camshaft cylinder head design can use straighter ports of more advantageous crossection and length. The OHC design allows for higher engine speeds, which in turn will increase power output for a given torque.
So OHC can provide greater power. This does not necessarily relate directly to speed, but a reasonable assumption is that yes, you would expect it to be faster.
DOHC, or dual overhead camshaft, allows even freer gas flow so can provide even higher power output.
OHC is easier to maintain, as more components tend to be external to the engine block, so they can be adjusted and maintained, as well as checked for problems.
These days there are very few OHV engines anyway.