AGI uses 16c 168x300 resolution, while SCI uses better (up to Vga, when it's out) AGI is a really simple language. SCI is a bit more difficult and is said to look a lot like C(++), but SCI allows more things to do than AGI. If you want the possibility of graphics like 'Laura Bow' or 'Quest for Glory', go for SCI. If you want the possibility of graphics like 'KQ', 'Gold Rush' or 'Manhunter', go for AGI. Since you know C++ and assembly, I guess looking at the difficulty of the language doesn't matter for you.
-Kon-Tiki-