Former HD500 owner here...I don't currently own any modeling gear. As I'm still searching for something that sounds good and has an easy to use interface.
I liked my HD but found it frustrating to dial in and was never willing to touch the settings in a live or rehearsal situation. It's just not a user friendly, or intuitive device. However, I did manage to get a few very good sounding patches set up.
I tired the Amplifire - I actually thought my old HD was better in many ways so I returned it. The Amplifire had a much easier to use interface but to get any decent sounding amp models you had to buy and load IRs (complexity increase) and the FX were very weak so pedals were required. i don't get the appeal of it.
I was going to get another HD then I saw Helix. I wil wait until I can try one out, but if it sounds at least as good as the HD, has a better interface so I can make changes on the fly (aka Live or at rehearsal), I will likely buy one.