[PC/SR]

Everything else being equal, an OHV will produce more HP than a flathead. How much, in actual practice it depends on a lot of other things in the set up. My flathead dynos at 114 HP at 3600 on a 6.8 CR. Better than a lot of OHVs, less than a lot of OHVs. A lot depends on the cam, carbs, bore, ports, valves, etc etc.
Whether to spend the money on an OHV depends on what you want to use it for and how much perfromance you want. For ordinary cruising or some racing save the money and get a good flathead setup. For all out performance get an OHV.
As a very general proposition, in the FAST hillclimbs, the fastest flathead is about .5-1 second slower than the slowest OHVs. (There are exceptions.)
Personally I like flatheads, but there is a "cool" factor in the OHVs, which has nothing to do with money or performance.