The only off site I would even consider are Ace, Advantage, Routes and Fox, probably in that order. They have at least somewhat of a history. Not all of it good but that's still more than what I know about the others. I had a booking with Fox that I held for six months until National finally beat it. I had considered Routes last year, they are somewhat popular in Canada.
I just checked with Autoslash and for my dates next week the best price is $267 for Ace, the cheapest on site is Thrifty at $358. My National rental that I have for $295 is now $600.