I was looking at the same two exhausts, in the end I went with Invidia but I don't have HFC. I don't think the Invidia is too loud and sitting at cruise speed it is the same as OEM, you only hear it when you put the foot down. At the time if I had picked the Miltek I would have wanted HFC to make the noise I wanted but I feel I didn't need it with the Invidia.
Hope this helps, but it is all personal preference so you're doing the right thing hearing them in person.