I think pokerstars is the best for beginning mainly because of the huge selection of games you can play the the small buy-ins.
Also, players are just terrible at micros on any site. You either have complete nits who you can run over or just players being to aggressive which you can take advantage of.
Sure, I think pokerstars has the better players on compared to smaller sites and much more regs even at the smallest limits but where there are more better players, there are more fish too. At micros, it shouldn't be a huge difference in skill set but software/playerpool/games just makes poketstars an easier choice for me.