Most noticable differences are paint, front bumper, and wing. '03 came in Black Mica, and Spicy Orange Mica. '03.5 comes in Titanium, and a shade of Yellow(never did hear what Mazda's name for it was). '03 uses the same front bumper off of the MP5, '03.5 uses a completely new front bumper that does not have fog lights. '03 has the same wing as the MP3, '03.5 has a Lancer type wing(Which looks a hell of alot better IMO).
Other than that, I'm not sure what differences there are.
Basically it looks less like a 17 yr old designed it. They took all the tacky aftermarket stickers and logos off it. Everything now says MazdaSpeed, instead of RacingFart and whatever that other crap was. HUGE imporovement.