I would rent a full size and then hope for a PC upgrade to something like an Avalon, Town Car, Caddy etc. I find upgrades to Prestige cars few and far between. Certainly not as often as you seem to get them in YYZ! My recent experience has been that when I reserve a midsize I often get a midsize, maybe a fullsize, but when I reserve a fullsize I get upgraded more often. YMMV of course.