I've visited both the Blue Lagoon and Myvatn Mineral Baths.
Blue Lagoon is without doubt the classier of the two ... great changing rooms, café, shop etc. Myvatn has a much more gravelly floor which didn't help my foot problems ... and the silica mud at the Blue Lagoon was like heaven to them! Blue Lagoon is larger and has much more 'usable space'.
Whilst Myvatn is simpler, I didn't really appreciate the experience as much ... but then I don't get much opportunity to go to this type of spa at home!
Go to the Blue Lagoon. I don't think you'll regret it.