Tough call. They're both really good receivers, and on the board we probably have a 50/50 split between HK and Denon. The Denon used to have more features than the 630 but the 635 is directly comparable feature-wise.
On paper the Denon has more power but the HK seems to be spec'ed even more conservatively and seems comparable in practice (assuming the 635 amp stage matches the 630).
I like HKs so would probably go with the 635 myself but if recommending for someone else I guess I would have to point out that while the 630 and 3805 are both well known quantities the 635 is brand new and doesn't have the same track record. Again, that wouldn't stop me but it's something to think about.
Everyone on the board with a Denon is going to be thinking "get the Denon" and all the HK owners will be thinking "get the HK"
You can't go wrong either way.
