The V3i is certainly prettier, but overall, I think that the CU500 gives you more bang for the buck. Stereo bluetooth and outer music controls alone make it a better deal. Throw in the fact that it has very fast EDGE (vs GPRS on the V3i) and can actually stream video without a 3G signal, and it becomes almost a no brainer.
That being said, if you like to tinker with your phone and enjoy modding, they the V3i is probably for you.