Vocaloid as a software package is more professional based while UTAU is made by amatuers for amatuers. Vocaloid doesn't let you record a vocal voicebank for use and it gains all of its vocals from 3rd party studios who produce them under license. The quality of vocal results varies in UTAU from crap (so much background noise, poor microphone, etc) to "near Vocaloid" quality. In the case of Vocaloid all are recorded in studio spaces under a higher quality standard and even the worst Vocaloids for quality (e.g. Sonika, Kagamine Rin/Len) are able to be used with some practice and experience a lot easier than the worst UTAU. Vocaloids also are recorded under a scripted piece given to the studios by yamaha while the method of recording procedure varies for each UTAU.
UTAU has the advantage of being a freeware software that anyone can adapt to their own use, each vocal is normally passed around freely and is subject to the creators license. Vocaloid is licensed and you cannot use Vocaloid voicebanks with any other software BUT Vocaloid. You have to pay for it, but it is professionally supported. Vocaloid generally sounds more realistic and matches a human's vocals better then UTAU. However, a well-made UTAU voicebank, when used correctly, can argueably sound just as realistic as a Vocaloid, although it is more difficult to make a UTAU sound realistic than it is to make a Vocaloid sound realistic. There are hundreds of voicebanks that one can download for UTAU--1,000+ vocals were reported. The UTAU fandom is overall smaller and there are not as many UTAU songs overall as there are Vocaloid.
Overall, Vocaloid is the easier-to-use and higher quality software, but if you're penniless it's UTAU for you. There are some good voicebanks for UTAU, but finding them is much harder. The Macne family are a good example and can be used with UTAU due to being open license; you must own the original though do take note. There is a Mac version of UTAU as of mid-2011 which Vocaloid has yet to expand into. The Vocaloid software is also about to spawn into version 3 with all Vocaloid 2 software being able to be imported into version 3 though details remain unknown at this point. the original Vocaloid software banks are being left behind.
VOCALOID also has more parameters that allow editing the vocal in different ways and VOCALOID generally produces different results from UTAU. It should also be noted that there are other vocal synthesis programs out there like CeVIO, and people have also managed to make VOICEROID sing.