We're using both, and they are not all that different when it comes to features. Notably, Oracle supports PVM for Windows guests, and can be managed from within Oracle Enterprise Manager.
If you are planning to run Oracle databases you may want to go with Oracle VM to avoid licensing issues (such as having to buy a database license for every cpu in the cluster) and to have a single point for support.
edit> There's one significant difference, come to think of it; The management tools for Oracle VM does not support snapshots, although you can probably work something out on the storage level (LVM, NAS or SAN).