Or Kings Gambit theory.
If you had to pick ONE b4 or f4 !?

#f4forlife
Only 11 Bird games on chess.com.
Well:#f4forlife...???????
Rip, exposed

Or Kings Gambit theory.
True, but this raises the question of whether it is safer for white to respond to 1.f4 e5 with 2.fxe5 (From's Gambit Accepted) or 2.e4 (the King's Gambit by transposition). I played the King's Gambit early in my chess career, and often got into trouble. Later I played Bird's opening for a while, but did not do too well with that either, often struggling for equality. Although 1.f4 has some surprise value against lower-rated opponents, it seems to throw away white's opening advantage.
With all that said, on the rare occasion that I still play 1.f4, I would rather respond to 1...e5 with 2.fxe5. I don't like offering gambits, and I hear that the best way to try to refute a gambit is to accept it.
I would prefer 1.f4 since it controls a center square, and it can transpose into very respectable openings. The reply 1...e5 is not a worry if you know From's Gambit theory.