1) Three axis electronic compass will give you the proper direction regardless of what position you hold the GPS in.
2) SiRFstar III chipset acquires and maintains a signal lock in urban or densely covered forest environment also its faster time to first fix (TTFF)
3) An external antenna connection.
4) Uses AA batteries as opposed to the AAA or rechargeable lithium-ion.
5) Backlit display can be turned off and it also has a sleep mode for the device.
6) Uses both National Geograph Topo (USGS QUADS) for better topography display and the Mapsend 3. You can toggle back and forth between the maps if needed.
7) Can have photographs attached to your waypoints or geo-cache regardless of which model. The touchscreen 2000 will let you take photos and attach them to your current location for later use.
8) SD card is on the side and not under the battery.
9) TrueFix 14 channel supports both WAAS and EGNOS (European geostationary navigational overlay service) the same as the 600 no real difference here.
10) vantagepoint software which you can actually download and try out without having to purchase.
11) overall it's more rugged especially the center toggle also the screen is inset for protection.
12) multiple user profiles to rapidly change your settings
If you're 600 fits your needs then there's really no point in getting any other GPS
The GPS handhelds I have are the SporTrak Topo, eXplorist 600, Triton 2000, Delorme PN-20, Garmin eTrex Vista Cx, forerunner 201 and 305. also Onix 400 but returned it.
Let me know if I can be of any further assistance.
GW