Mag 525 is the first mag reel brought out by Penn - braking is controlled purely by magnets (ie no brake blocks) (of course oil thickness and line level also play a part in control). Mags - of which there are 4 - are controlled by a slider on the left hand end plate. Each click of the slider from the slow to fast position makes the reel progressively faster.
Mag525T, is a varation of the Mag525 - used for tournament casting. It has one magnet and is controlled by a large 'screw' on the end plate which the caster adjusts once the lead has been launched toward the horizon.
Supermag is the Mag525 with higher quality bearings and 6 magnets instead of 4 (also has a gold coloured handle to match the spool). The increased number of mags gives an option to run the reel slower than the original with the same oil/line settings.
I am a big fan of the orig Mag525 (havent tried a supermag yet). They have everything for me: Cast well, can handle a wide range of scenarios (clean to semi-rough ground) great winching power and a ratchet. They probably arent as refined and don't cast as well as the Abu Mag Elite, but as far as fishing is concerned I think they win on just about every aspect.
Having said all that, I still love my mag elite... well who couldn't