This isn't really an answer to your question as it's a bit subjective and depends on your comfort level with the technologies involved. But here are a few things to think about.
If you need to be able to do live migration then I think this becomes more a mute point. To support live migration you'll need some sort of shared storage that all the XEN host machines can see. You can use NFS or iSCSI for this.
Both Ubuntu and RedHat/CentOS are unfortunately currently moving towards KVM. Ubuntu's support for Xen as a Dom0 has been phased out in the newest version of Ubuntu Server in favor of KVM and CentOS's Xen is behind the Xen development by a couple of versions. Both OS's have decent support for running as a DomU.
That said currently our virtualization is done on Xen running on CentOS and it works well. Although I would love to have a few of the features available in the newer versions of Xen.
I know of a lot of people in the Xen community are running Dom0's in Debian and Gentoo as well so there are other choices. A lot of people will compile their own Xen kernels rather than use the ones from their distro's as well to stay in step with Xen's development.
The beauty of virtualization is that you can try different Xen hosts and figure out what works best for you without necessarily having to do anything to the guests. However if you're using file based DomU's I do believe that Xen Server uses VHD image format by default where the open source version uses raw image files and qcow files. So some care needs to be taken when picking a format to use for your images. There are utilities that can convert between these formats out there as well.
EDIT: After doing some reading it does appear that Ubuntu has decent support for Xen still. Xen 3.3 hypervisor and Xen kernels are in the repository. For more info see https://help.ubuntu.com/community/Xen