If you only have one day I recommend you spend it in the Disneyland park. The new Walt Disney Studios has only been open a year, and is still very small with few attractions and not much themeing. You certainly can't compare it to MGM.
Although the Walt Disney Studios is not yet a great park, the Disneyland park at Paris is without doubt the best Magic Kingdom that Disney has yet built! The themeing is rich and wonderfully conceived, many of the classic attractions (Pirates, Big Thunder, Haunted Mansion) have been reworked and improved on, and Space Mountain has to be ridden just so you can see how tame it's American cousins are!
Tips? Make sure you get to the park just before opening time as the queues for the main attractions built up quickly. Several of the attractions have FastPass (Big Thunder, Space Mountain, Indiana Jones, Peter Pan) so make good use of those. Of the major attractions Pirates and Indy tend to have shorter queues, so you could leave those until the middle of the day when other queues are 60 minutes+. If you have time try to see the Tarzan show - it's a lot of fun. Finally, check out a few of the trip reports on the DLP board. You should get lots of inspiration and tips reading a few of those.
Regards
Rob