If you are comparing strictly GSM... in Markham Fido does have some dead spots, but the coverage would be much better than Rogers GSM. For some reason Rogers is great outside but once you enter a building Fido gives me more signal. This is on the same unlocked phone.
If you are more specific about where in Markham, people may give you a better idea.
Well, now I am in Markam and I did purchase the 3390. Coverage here is pretty good. I am close to 16th Ave/Fincham Ave. Down in Scarborough, the coverage is okay, too. I bought it at the Future Shop in Markham and the sales rep was very nice. She even "warned" me that the coverage indoors is not very good. It indeed is not spectacular but I don't think I missed any calls yet.