I would say get the star mags, just because you get even more ammo than the MAG mags, though I will say that when I was looking for M4 mags, my final decision was MAG's. (even though I'm probably waiting until spring to buy some with a few "M4 using buddies" on the field).
Plus there are plenty of good comments on the MAG mags.
I can tell you one thing about the MP5 Hi-cap though.
A while back I purchased the CYMA CM.025. So far, it's probably one of the worst shooters I've ever seen, though hooked up to a little battery pack I made, had an insane rate of fire. And the cheap Hi-cap made by CYMA kept feeding, and I was very satasfied with it. I'm actually still holding on to it if I/anyone else on my team happens to buy an MP5. (I'm thinking about the AE MP5 myself, the more I look at it, I find it to be a good buy.)
So my point is simple: That hi cap looks a lot like the CYMA version, and since the el cheapo CYMA mag worked great under a high ROF system, that one should do you just fine too.
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