There's a strong case for not using anything (Basic & Advanced D&D never needed minis) but if it's a choice between minis & counters for NPCs, use minis. As a player I _hate_ getting killed by a piece of cardboard! My GM had been using minis, but she switched to counters. If that piece of cardboard had been a mini I might have realised it was an Orc Sleepless (Commander) Ftr-6 and not just an orc mook Ftr-2 before it Full Attacked my wounded female Fighter Zana into salami slices, *sigh*. Best PC I ever had. 
Minis definitely are better.

Minis definitely are better.