Well, I've seen APIs of both and Allegro's IMHO easier to comprehend. Plus need to carry just one dll around with your app instead of SDL_mixer.dll, sdl.dll, sdl_ttf.dll, etc. is also a feat. Just read APIs and way of doing things in both (may be even C examples - they're surprisingly easy to translate into Pascal) and you'll get to same conclusion.

But this is offtopic, so I'd like to get this on PM if you have further questions/discussion/dead threats about the subject.