Title says it all. However i have to enter at least 30 characters.
Percona's Xtradb has some improvements -- mostly in scalability. The changed the Mutexes, which held back effective use of multiple cores.
IMHO there should be no real difference. However, let me add this:
I actually wrote something up on this a while back
In my posts, I mentioned InnoDB 5.1 Plugin outperforming native InnoDB 5.1, 5.5, 5.6 WHEN LEFT UNCONIGFURED !!!
When it comes to XtraDB va XtraDB plugin, I expect about the same. While left unconfigured, the two may or may not be the same or one be better than the other. Yet, you cannot get the most out of InnoDB or XtraDB unless you configure it to be.