You can make a loop in Studio...but it's just not nearly as user-friendly as GB or Fruity Loops. However, you can export your loops from GB to Studio for more control and features.....
I've been using Logic Pro 8 since Saturday. Got most of the main stuff sussed out already and I'm almost finished with a new song. The mix sounds great, minus vox and lead part. A noticeable improvement over GarageBand, in fact this program is GaragBand on steroids. All the things about GB that were frustrating me are very nicely addressed in Logic, and I think it takes a lot of flavors of plug-ins too. Marty can set me up "under the hood", so to speak. I think the only better rig I could use would be Protools. The interface is nice, pro apple standards.
Logic Express 8 for me. It does have drum loops, and all types of instrument loops you can cut and paste. Also, full midi instrumentation. I will lay down my bass track to my group's compositions using a midi keyboard first to snap the timing up with a drum loop (nice crisp rythym track instead of a stale click track). That way if we change the tempo when first recording the song, all the rythym tracks go right to that new tempo. I'll record my analog bass later. Logic Express 8 is very intuative, easy to pick up. I used digital performer previously, and the learning curve on logic is not as steep. $200 for Logic express is a steal also. Here is a good series to learn some basics and the tips are free.