GBA: Great
SNES: Good on most games, games like Mario Kart, Starfox, Super Mario RPG are slow
Slim: Smaller (not width wise, just thickness), and lighter. I've held one and each hand and the Phat compared to this is a brick. Only thing is: no IR port (can't turn off random TVs with homebrew) and most 1.50 homebrew won't work (Most have been changed to work or work with eLoader), can use Skype, has TV out.
Phat: Larger (again, just thickness), heavier, has IR port, all homebrew works, no Skype or TV out.