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If you are going to make this site enormously popular then owning the .com will allow you stop the loss of type-in traffic to the .com domain. For popular .org .gov .net and other domain extensions, lots of visitors instinctively type .com as the domain.  I do it often myself when attempting to visit some .gov and .org sites.  I am usually surprised at what I find there. I know of a couple .com domains that get thousands of visitors per day because visitors don't type .gov as the extension.

Istvan makes a number of good points and Matt Cutts has certainly alluded to a loss of some link juice when using 301's, although Google's official line is that there is no loss. I'd not seen the 15% number before, which is certainly high enough to be discernible above the 'noise'. I support his contention that, in terms of getting existing inbound links repointed, it's best to focus on the few high value links and then look for new links driven by quality content. This has the double benefit of cleansing some of the now-devalued link types, whilst appealing to Google's measurable preference for 'fresh' links.