As already stated, two different types of rifles. The HK without a lot of mods is cumbersome. It is more precise though than a standard AR. It's an HK if that means anything to you, and it's non restricted. It's more of a varmint/target shooter out of the box without the mods. With mods $$$.
The AR-15 is lighter, quicker with lots of add ons and is cheaper. It's a decent rig. The only issue is the restricted status.
Now... Precision AR-15s blurr the line. We're still waiting for the Stag Model 6 which would be a fairly affordable target shooting AR-15. The only issue again is the restricted status. If it wasn't restricted, this would clearly be the rifle to get. .5 moa guarantee with match trigger. I'm still interested in one though for range shooting.
So ask yourself what type of shooting you plan on doing? Also what do you plan on doing with the rifle?
If you want to shoot varmints and are looking for a non restricted accurate rifle then the HK is a consideration. But if you're planning on changing anything on the HK, such as the stock or add the built in bipod, then a Swiss Arms Target is another consideration.
You've got lots of research to do, and some sole searching to figure out what you want to use the rifle for.
Good luck and let us know what you chose and why.