I'm not too sure about the availability part of your question, but I would check with your local Avnet person. Both devices are fairly new, so they should be around for quite awhile.
As far as the differences, it really comes down to what you really need:
5V I/O or more than 8 ADC channels -> use the AW
down to 1.8V, lower power -> use the GT (or GTA for more RAM)
The other consideration is the prototype build. If you need to build it yourself, then the GT is available in a DIP package. The AW is QFP(tough, but can be done by hand) or QFN(good luck with this one).
Just my couple pennies worth.
Brian