I personally like Guelph, due to its suburb setting, and proximity not only to the GTA, but also to the KW region (it is located in between GTA and KW). Living in Guelph also means that you can still work in downtown Toronto, though the GO train ride can be pretty long (
about 1 hour and 45 minutes one way).
If you are interested in IT, and don't care much for city life (then again, KWC area is only 90 minute drive away from most of the GTA or downtown Toronto), then you would like KWC much more than Guelph, since there are a lot of more tech jobs in KWC than in Guelph (not to mention the presence of University of Waterloo, and a very strong startup scene in Waterloo).