The Yamaha IMO is a much nicer unit. It also weighs more. Not by much, but that should say that power supply is beefier. The HK weighs 31lbs under the Yamaha at 34lbs.
IMO I also think the Yamaha is a cleaner look than the HK. With the door that hides a lot of buttons. But the HK is a very nice looking product compared to their older lines.
Once again THX is a certification, as 98% of the people here would agree. Honestly the 2% that say you need THX, well I would go as far as saying they do not know what they are talking about. There are 4,000.00+ units that are not THX certified, but would easily pass the certification process. Once again the savings of having it certified would be passed on to you in the form of more money for that logo.
BTW, I am a THX receiver owner...... But it should not be the main part of what your decission comes down to. The power specs on the HK are probably a very true power. And very clean for that matter. However as stated before, even with my 1015tx that has a higher THD. I by no means (and I'm not alone) hear any distortion at super loud levels that I would only go up to once in awhile (just for testing purposes).
If you want some easy opinions and not explanations, then mine is go for the Yamaha out of those 2 choices and be happy. (I'm positive you will be happy)
That is unless you can figure out how to get a 1015tx.........