I've owned both units , and I would advise that you read up on both units as much as possible because although you can get very similar/identical sounds out of both, they are very different.
Personally I find the firehawk a joy to work with (I'm really into using my phone to edit) whereas I found the hd500 felt like a lot of work to get good tones, the firehawk is much easier to use but less capable, the firehawk can't do dual paths or 2 amps at once for example, I don't need those so the firehawk wins for me