Well I'd say the difference is quite small delay wise, but you have to look at the ammo capacity, too.
6 instead of 5 - which could come in quite handy when you play a lot of CWs (and rooms without instant reload / infinite ammo).
Donation Shotgun I/II - 100 coins, 900 delay, 5 clip.
Black Infinity I/II - 150 coins, 900 delay, 6 clip.
Black Mamba I/II - 150 coins, 850 delay, 5 clip.
Any donation shotgun for 200 coins (e.g. Genesis Revenge, Duelisto, Pony, Mayuri) - 850 delay, 6 clip.
