I have both a GNX2 and a Tonelab.  If you are looking for tone, meaning your primary purpose is an amp sim, then get the Tonelab.  If you want to use it more for it's effects (i.e. you already have a good sounding amp), go for the GNX.  The GNX takes a lot of tweaking (and a lot of learning) to get a good tone.  The presets bite, but a good tone can be had if you really work at it.  Having said that, it might take a week or two of doodling till you real get a feel for the unit.
If you already have a computer with recording software and such, don't waste your time with a GNX3 or 4.  The don't add any new tones as far as I'm aware of, just a drum machine and stuff.  If you go for the GNX stuff, save your money and go with the GNX2.
On a side note, Digitech lied when they said that the GNX is MacOS compatible.  It's not.  Considering how many Macs are used in the recording industry, it makes you wonder where Digitech sees their products (i.e. pro vs. hobbyiest).
Another note is that Digitech seems to have a problem with QA.  My GNX2 is dead (I need to send it off before the warrenty runs out).  If you peruse the Digitech boards, you'll see that a lot of people have problems with the units failing (e.g. continuously rebooting).