ok so i use ak-47 like as if it's the only gun i use in r2da and r2d... and well, ak47 is probably the best jack of all trades gun: moderate damage, best accuracy (of all automatics), far range, fast firerate, fast reload speed, good clip size, amazing compatibility with clip enhancing armors, and so on that makes it very even.
flamethrower: cant hurt 2 types of zombies, very close range, slow reload time
i would go for ak47 because it is able to hurt any zombie at any range except extreme sniper ranges with good accuracy, so you can keep your distance from danger unlike the flamethrower where you have to run into the danger to actually kill your zombies
but thats just me, if you want to have a balanced gun use ak47, and for an offensive gun, use flamethrower.
AK-47 is much better if you're going for med to close range attacks. Flamethrower is only good for close range but a great farming tool. So I would say Flamethrower since then you farm more to get an AK-47
Theres a lot of different opinions being posted, I would say it depends on your purpose. If you're looking to really grind on singleplayer, Flamey is FANTASTIC. If you're looking to grind on say Void (Which I have found is more effective) probably AK-47. TDM or FFA? Dosen't matter which, both are effective but its down to your playstyle. With mine I would go flamey. Looking to not grind but play and have fun while leveling? Flamey. Overall, I would agree Flamey is the better choice due to the much higher zombie kill potential (AKA More zombie kills per round are possible than with AK.)
I got flamey first.
Edit: Get flamey to grind on singleplayer for cash and you can get the AK in no time.