You know guys, when someone asks about getting one or the other, they usually want you to choose one or the other unless they give a "all of the above" option. Go with Space Invaders. It's quite epic.
Both - but in a desert island scenario - Space Invaders- working with your music essentially gives you as many levels / games as you have songs..
Hmm...both are really great games. It really depends on you because if you like exciting, fast-action games, go with SIIN, but if you like adventure games and appreciate the unique art style, go with Spider.
I go with space invaders. Spider is a masterpiece of the iPhone, but space invaders has great concept and almost infinite levels, as many levels as there are songs on your iPod
You should get both eventually as they are both excellent, but spider is better than everything but real racing imo... go with spider