One of the jME developer blog has an interesting head-to-head comparision for jME, Java3D and Xith rendering the same Quake3 map. See few differences on sky color and here and there “shadow” differences on a ground.
http://www.renanse.com/blog/
Other than that, Xith were slowest of three. fps were taken with FRAPS tool, I dont know how it works but I consider it gives about the correct results.