M855 isn't an "all that and bag of chips" round. It has less than optimal terminals, and has accuracy inconsistency. 5 MOA ammo is a norm.
SS109 is a better round but not by much. If you have a budget, look in to Gold Dot, TBBC, Fusion, or a few of the bonded rounds. If I feel the need to shoot through a car, they would be my first choices.
If you just like M855, go with IMI.