GT-6 is a complete unit with expression pedals incorporates 30 amp sims and loads of boss fx in software. However it has reduced connectivity, because Boss don't do a internet download service for it in the same way Line 6 do for the pods. The GT-6 is designed for live work but sounds good in the studio too.
If it is the all in one aspect of the GT-6 you like as opposed to the modular design of pods with separate pedal boards etc then you might be interested in the PodXT Live http://www.line6.com/podxtlive/features.html
- this is pretty much pods answer to the GT-6. I would imagine it is better but be warned, as the software for uploading/downloading patches is still not available. Although I am sure it is only a matter of weeks before they sort it out.
POD's are very studio friendly and have lots of plus points for PC connectivity. A great many of the members here use pods to good effect. However there are a few GT-6 users also that get great sounds too. Both pod and gt-6 owners have access to thriving communities on the net.
Basically it is up to you and dependant on your requirements. Personally I use Boss/Roland gear but I have to say on the evidence of the pod users on this forum I think Line 6 have got it right and I would definitely consider getting one of their products in future.