which is better !?

Depends which mode you play as if you play ghost mode or knife only matchs a lot i would pick the axe if not pick the m4 custom.

It depends all on your preference. I would totally go for the M4 [because I don't generally knife unless I want to embarrass somebody] but if you play Ghost Mode and enjoy being a Ghost, you may not even want an M4 Custom, but instead want a permanent axe, so you do not have to keep purchasing temporary melee weapons.

You simply have to decide which one you want. If it was comparing two guns, the community could give you a semi-biased answer, but this is an argument between a melee weapon and a gun.

So my final answer?

If you play Ghost Mode BL more : Axe
If you play any other mode : M4-C

Hmmm... I suggest you to take axe as I'm a fan of GMs xD
Tho M4A1 cust is a goood gun