See I always get confused on why people call the 68 rounds low caps and mid caps....either way going for the 68 round is the way to go in my books, gives you a more real feel when you play, you save on bbs/ your force too if you only have 204 rounds on you, and you don't need to wind anything.
The problem with MAG and other mid caps are that they have to be maintenance every so often, or they double feed like the high caps. If you are willing to do so though then by all means get your hands on some. I haven't heard too many back reviews on silverbacks however I have only seen airsoftparts and airsoft club carrying them.
Overall if you lazy and don't want to maintenance your mags (google review or Mag and silverbak mags) get the low caps, if you are willing to though go for the mid caps and high caps.
Also try staying towards the companies like CA, TM, Echo1, KS, Silverback and Mag, all provide the best quality mags for P90s.