I'm facing the same dilemma, although for me it's a lot simpler. Since I live in Portugal, the best deal for an unlocked Nexus 5 is 400€ which is about 550usd. Also my galaxy nexus isn't laggy at all having a kitkat rom with fancy kernel. So, in my case, either the price drops a lot, my galaxy nexus breaks down, or I wait for another nexus iteration.
In your case, 5$ * 12 months = 60$ in savings on your phone bill. Fido Dollars seem to be something you've accumulated over time, and so the 50$ difference you have to pay is covered by the savings over a year. So if I were you I'd probably upgrade.