I can only speak to my VFC HK416. It's a wonderful RIF in terms of feel and build quality. As for performance, I'm not sure yet - haven't actually taken it out for a skirmish or shot more than a few BBs at proper range through it (I expect this to change this coming weekend).
From what I've seen of it so far, the recoil is amazing, but the power tends to be a bit variable. I suspect absolute performance will therefore be less impressive than the MWS.
I've also seen some good review videos, including Bada Bing's excellent direct comparison between this and the MWS. The MWS definitely wins out on full-auto as it's less prone to cool-down effects in cold weather. (My gun is great in short bursts at room temperature but I've not personally tried mag-dumps or properly low temp shooting yet)
I'd therefore pull this conclusion out of my ass, see if you like the cut of its jib:
VFCs can be seen right now knocking around in the classifieds for a nice discount on the new price. MWS's tend to hold their value better, I think.
That means you can maybe pick up a VFC with ~80% of the performance for maybe only 50-60% of the price. That might seem like a good deal to you - it certainly does to me - but if you want the best performance then you have to pay (over the odds) for it. So that's a decision for you.
The MWS is the better RIF, but the VFC (and no doubt similar guns from other manufacturers) will be better value.
Edit:. Having a squizz at the MWS thread suggests they're not immune from problems either. GBBRs are maybe a bit 'more art than science'...? ?