Hi Randy
We tend to split the disks between controllers and allocate all the disks (minus spares) to the aggregates on setup. I don't like to WAFL iron / reallocate if I can avoid it! I also don't like to put all the load onto a single controller.
As others have said, keep an eye on performance and move volumes around as needed to balance space and IOPS.
For your Xen NFS storage repositories, I would use several volumes. You might lose some de-dupe efficiency but you'll gain a lot of flexibility. Create a separate LIF for each volume and home the LIF with the controller that has the data. If you need to move one of the volumes to the other controller's aggregate in the future, you can move and re-home the LIF to keep traffic off the cluster network without affecting the traffic to the other volumes.
Good luck
Steve