Well, I did some research on both of them and took the reviews of CNET. Despite the fact that the M1330 is 0.1 higher than the M1530, I'd go for the M1530, one reason because of the better customer satisfaction. It also has a larger screen. The M1330 is a more portable laptop, not as customizable for gaming. Don't take my word for it, see for yourself.
CNET - Dell XPS M1530 CNET - Dell XPS M1330