I usually go for full speed in HD Remix mode. I’m fairly certain that most online matches are played at full speed, unless there’s a desire to go for a more slower retro sort of feel.
As for playing in Remix mode, there’s a certain air of finality with playing this way, the definitive Street Fighter II experience. It’s like, “Yeah, there have been about a million other versions of this game, but this is the last one, we promise.”