Basically, Bell/Telus/Rogers are the "big three" in Canada. They barely compete with each other, and it's believed that they actually conspire to try and take all of your money (because that's easier than fighting over it). Their networks are all solid. Some people say that company X has better/worse signal/price/customer service, but they're basically just the same.
Wind and Mobilicity are the new upstarts, and their prices can't be beat, but their networks are weak and they're basically confined to Vancouver/Toronto. The big three want them dead, and Mobilicity is already doomed, so if you're considering one of them, go with Wind. Also, you will pay more for the phone.
Koodo/Fido/Virgin are the "fake competition". Koodo = Telus, Fido = Rogers, and Virgin = Bell. You can get all of the benefits of a powerful network from these guys, with a lower price (although not quite as low as Wind).
So basically, if Wind works in your area, and if you want the lowest possible price, and you're willing to put up with some weak/spotty signal, and maybe if you like rooting for the underdog, then you should get Wind. If you would rather pay a bit more for a better/more reliable signal (and have that increased price offset a little by a cheaper phone), then you should go with Koodo/Fido/Virgin.
Edit: BTW, Telus has a reputation for being the "least evil" of the big three (with Bell being the "most evil"), and I've heard people say that Koodo is the best of the fake-three.