The way I see it, TPC has more tracks, more tutorials and more attendance. It is also a lot more expensive (which might or might not be a factor if your company is paying for it).

Remember though that days only have 24 hours no matter which conference you attend and how much you pay for it.

So if you are interested in some of the other tracks, like XML or Apache, then definitely choose TPC. YAPC is smaller, focused exclusively on Perl, there are definitely less newbies there (although I noticed last year that apparently, with the downturn, only senior programers were allowed to attend TPC). It is also, I think, more friendly, especially to newcomers. It is very easy to talk to people at YAPC (especially for monks).

Being at YAPC makes you feel like you are part of a small, friendly, closely knit community, while being at TPC makes you feel like you're part of a powerful and important force that will change the World. Or something. And there is beer involved either way ;--)

You pick!