AK GOLD!!!
I have both weapons so take the advice from me,
Ak Gold is unique, it excels in both power, and ammo and its different from the other ak's where as m4a1 gold only has more ammo but its accuracy/recoil is worse than that of regular m4a1.
It depends, if your a close range kind of guy, who loves mutation, gm (the ak noise will affect others), td then pick ak gold.
If you like to snipe your enemies off with rifle then pick m4 gold.
My suggestion = Ak Gold because its more unique than m4 gold.