mostly little bells and whistles. The newer PN series allow direct import of .gpx files, which isn't really that much of a time saver since you have to go through the software anyhow.
Increase in processor speeds, which is nice if you use aerial images, though the PN30 was a big step up from the pn20 in this regard and totally usable in a vehicle. Screen features are more decorative. Nice thing about the PNs is you can use AA batteries. This is a big deal if you backpack and will be gone a while.
A IT-20 would have to be paired with a laptop or other device to display your location, so it's use is limited outside a vehicle.
I still use my pn-30 every once in a while. Iphone running motion x is my preferred solution unless I'm backpacking.
Depending on how you intend on using a GPS, older units are fine. The only limitation you will eventually run into is a lack of software support for their mapping application or the device's inability to eventually use their mapping software when it upgrades. This can be solved by keeping an older computer around.