BULLET PUNCH
PROS
- accurate
- STAB
- easier to teach than meteor mash
- priority
CONS
- weaker
METEOR MASH
PROS
- stronger
- STAB
- small chance to raise attack
CONS
- inaccurate
- harder to teach
In my opinion, if it is not mega, go for bullet punch because it has very bad speed. If mega, run meteor mash because it is faster than its base form. [note: the moves are easier/harder to teach in my opinion]