Snots are small and playful and like excited puppies. No love of fighting as such. Just giddy and daft. Vs a human not much of a threat. 50 of them trained to stab you with sticks pretty nasty. Same number having been through a tunnel through the warp and berserk with terror materialising inside your vehicle or armour, or even you, really nasty.
Grotz are mean and nasty. One on one with a human they would scarper. End up in the hands of a few of them they would be horrible. Stats wise on the table top similar S and T to a human really. In the old days a stat of 3 was a trained soldier, so
BS or S or T of 3 meant you were tough and pretty good. 4 was elite. 2s were what joe blogs off the street would be. So a battle hardened (

) grot would tussle well with a average non military human.
See these quotes from waargh ORKS.