F6B:
Mechanically reliable
Doesn't leak oil
Does not overheat
Faster in acceleration
Minimal maintence costs
More value for initial cost
HD:
Not mechanically reliable
Leaks oil
Overheats
Slower in acceleration
Major lifetime maintence costs
Less value for initial cost
That's it in a nutshell. More may add to it. We do have a lot of HD ex-riders and concurrent F6B /HD riders as well on the forum. You'll get some good further comparisons.