Go with the cordless. The price difference to include two batteries (and I guess a charger?) makes it a reasonable bargain.
Just make sure you're not actually being sold one of those terrible PSC 400 ones, they're probably one of Festool's worst products ever.
I had the PS 420 first and then upgraded to the PSC last year (because the cord is seemingly ALWAYS 2 inches too short at the end of a sheet). No difference in power and handling but not having a cord stick out the back makes this SO much more comfortable, I love it (well, as much as one can love the 420 series with it's quirks, I guess... [unsure])