It depends what you really want to do.
Both are rather easy graphics libraries, but they have a few major differences.
For example, if you ever plan to incorporate 3D into your programs, then you should go with SDL because it has very good support for OpenGL, while allegro does not.
On the other hand, if you only intend making 2D graphics, then both allegro and SDL are capable of almost same things. SDL makes collision detection easier, and has some nice extensions to do sound, networking and other useful things. SDL also has more tutorials and books.
You see these are similar libraries with similar capabilities, and are both cross platform. Its very hard to say which is better, so it would be best for you to just write a few simple programs using both, and see which one you like more.
Personally, I would always go with SDL, because I like how it is written, and I like the OpenGL support which can be used to take your programs to the next level (even 2D programs might benefit from OpenGL features to add particle effects for example).