Did a little testing on Flickr and my own on-site gallery (Gallery 2) and determined that IPTC kicks butt.  Both applications pick up the keywords and caption.  The one caveat is that they don't keep the information in an image when it is processed.  For example, if you download and save any of the resized versions instead of the original, it won't retain the IPTC info.  A little annoying, but not a deal breaker. 

Of course now this means I get to tag and re-upload all my photos everywhere...