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
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.