The Swiss Arms blows the HK out of the water, hands down! I own Swiss Arms rifles & plan on buying another. I also own a number of AR-15s which I like very much. However, no question about it, the Swiss Arms is a better built, more robust firearm. If you want something to hang all kinds fo doodads and gizmos off of, the AR is most definitely for you. If you're looking for a no frills, completely dependable and accurate out-of-the-box rifle that you can still take coyote hunting, then the Swiss Arms is for you.
I love my Colt, STAG & LMT AR-15s, but I love my Swiss Arms more. As for the price of magazines, while I'll admit I'd love to see the Swiss Arms magazines drop to US AR-15 mag prices, I know that's not going to happen. That said, one of the great weaknesses of the AR-15/M-16 platform has always been the magazines. They're fragile & easily subject to damage in a combat environment. The Swiss Arms mags are more robust, even though they're polymer, plus they have the added advantage of being able to snap together.
Some will argue that the AR mag change is much faster. Maybe for newbies to the Swiss Arms/AK style mag changes that's true, but with practice, you can come darn close to matching, if not exceeding those speeds. Plus, you get a much more positive lock-up than you do with the AR platform.
In the end though, this is one of those debates where there are no "right" answers. Kinda' like the age-old Ford vs. Chevy or Mustang vs. Camaro debate. Buy what feels best to you, afterall, you only have to please yourself!