>>1
Having used both:
Allegro:
Great for making DOS-style games (with DJGPP!). Lots of utility functions (but graphics functions are designed for 2D software blitting). Probably more fun.
Maybe you can use OpenGL now, I haven't used Allegro in a long time.
SDL:
OK cross platform wrapper for OpenGL games. Only basic very low-level sound functions (but there's SDL_mixer or some other library for that). Under LGPL, which sucks.
Of course, for a larger project you can just write platform specific code for windowing an input, and as long as you use OpenGL porting shouldn't be too hard.
>>5
WTF are you talking about, the basic SDL API is dead simple (although Allegro is maybe the easiest C library ever). Go look at wxWidgets or something if you want to see a real sepples API. It's very common for C libraries to prefix function names to prevent naming conflicts (libpng, etc). CamelCase is also common (Windows and Mac C APIs, etc).