I just prefer Plaxo. Hey I am a member of both!
LInkedin is described in Wikipedia as ‘a business-oriented social networking site’ whereas Plaxo is described as ‘an online address book service’. Both interestingly (if not unsurprisingly) are based in Mountain View and more significantly both are at least part funded by venture capital specialists Sequoia Capital.
My contention is that they both deliver an excellent service however right now Plaxo, with only 15 million users compared to 21 for Linkedin, offers me a more useful set of capabilties. Yes, they do overlap, but Linkedin appears to be operating a hybrid business service, a personal classified ad plus people finder whereas Plaxo is both keeping my contacts current (a la Linkedin), working off-line to keep my directories up to date and with a eye to the problems of mobile phone users keeping my HTC Touch Cruise aligned to all of my other contacts.
More importantly than this is the integration with Facebook, Flickr, Twitter, Blogs et al in a very clean interface making Plaxo ‘Pulse’ and invaluable tool for my daily work life. Linkedin seems to be like a perpetual ‘I’m looking for work’ classified without going the whole hog of placing your resume on Monster.
Both have a place and I could easily see the two consolidating as a point in the future, I just hope we end up with the Plaxo UI.