ECF a complete implementation of the RemoteServiceAdmin spec from OSGi 4.2, the CXF is currently being refactored to be compliant with the spec but is still valid distributed OSGi (the spec just mandates how they're to be managed, the key distribution stuff (what parameter to add to your service properties to make it remotable, etc).
ECF was, until recently, an Equinox only rOSGi implementation, but dependencies have been fixed and it should deploy onto Felix, Knopplerfish, etc with no problems.
ECF has a pluggable transport layer, see http://wiki.eclipse.org/ECF_Providers for more info (some providers clearly aren't applicable for distributed services). CXF comes from a webservices background, but has support for more performant means of distributing services too.
There was some talk about getting Riena to use ECF, not sure how that's gone.
If you've got the cash Paramus offer a very nice solution (http://www.paremus.com)
I don't like webservices so am somewhat bias towards CXF, also ECF is compliant (so you can move to another implementation later with no problems) and pluggable (so you could always write your own transport provider). If it were my call, and I had the cash I go with Paramus otherwise ECF.
UPDATE EDIT: I've just recently started playing with FuseSource Fabric, it's very nice (we're already using Camel with Karaf, so it dovetails there nicely). The RemoteServiceAdmin running over HawtDispatch is apparently very very fast.