if i were you, i would start off with agi for 2 reasons. its easier to start with if it is one of your first languages. and i like agi games better than sci.
on the other hand, if you know C, you may want to do SCI. Why? because there is not as many SCI games, and we need more SCI programmers. Thats whay i think.
im not telling you what to do. you do what you want. im just saying this in my opinion.