I've used the 252 for a long time, and it's nice. The 2520 has a boat-load of problems. You might want to take a look at the latest errata sheet for the 2520 before committing to that one.

If you can live with the problems/work arounds shown in the errata sheet, then go with the 2520. If not, you might want to shop around for a similar 18F series without a 16 page errata sheet..;o}