Personally - I'm a fan of OSGrid -there's a few reasons (getting regions up on something like slicehost is a good start - but I find the community is a lot more dynamic too), but the big reasons are that the OpenSim software itself is a
lot more mature - the current Rex server is about 6 months out of date (0.5.5 - which if you follow OpenSim is a very long time - both stability and feature-wise).
Rex is actually merging the other way rather than splitting apart - the big job that's being done right now is converting Rex to OpenSim+Rex.dll vs the current forked codebase. (full disclosure: DeepThink is contracting to Rex to help with that integration work).
This was a screenshot from the current 'modularrex' version we're working on as of yesterday (this is on a stock OpenSim with modularrex.dll installed)
I think Sakai's custom "3dxserver" will probably end up being just a distribution of the standard OpenSim software really, there's not a lot special about what he's doing as far as I can tell other than doing additional testing and throwing a frontend on it. Not disparaging what he's doing - just I dont really see much point myself since it just means additional maintainence and synchronisation work.