I have both the PN-20 and PN-40 and the major differences between the two is that the PN-40 with its dual core processor makes rendering of the maps and aerial images a little faster, 1 gig of internal memory and a altimeter making elevation readings in the PN-40 more accurate.
Delorme really amped up the 1 gig of internal memory. This is not really that wonderful since there is no file management system to move files from the SD card to the internal ram. Also, the PN-40 is still limited by the 64 file limit that was in the PN-20. Delorme did remove the upper limits of the SDHC so it now supports 32 gigs. They do fail to tell the users of the PN-20 that the PN-20 also supports 32 gigs with the latest flash update.
I do like the PN-40 for its speed in rendering images, but I really don’t see much of a difference between the PN-20 and PN-40 in the way of overall performance. If you are getting a Delorme for the first time, do get the PN-40. If you already own a PN-20, there is really no need to upgrade unless you really want to.