Looks like the Ultra 2 is advertised to support a faster CPU (Ultra 2 = 2100+, Ultra ARU = 2000+) Also, the Ultra 2 looks to have native USB 2.0 ports, while the ARU has both 2.0 and 1.1 ports (not that it makes much of a difference since 2.0 is downward compatible with 1.1). Although it seems that the obvious differences are few and far between.