The Fiesta is slightly nicer on the inside and drives better, but the transmission is terrible.
PlanB, where exactly in BC do you live? I can't remember exactly what Budget has right now as their compact car, but when I drive by a Budget branch, I can visually check. But I am thinking it is more likely you will get the Accent based upon availability.
Also, I am assuming this is for loss of use coverage. Is your car in a dealer or body shop for repair, or is your car totalled? If your car is in the dealer or body shop, ask the body shop to arrange the rental car; they will get reimbursed by ICBC as the shop has their own agreements with the local rental car agencies.
In any case, when you pickup your car will matter; don't go in the early morning; that's when most rental car places that deal more often with insurance, dealership and body shop rentals have the bulk of their cheapest cars, and the longest lines as they are getting everyone from these shops to their place to rent cars out. Go late morning or noon-ish; the likelihood of an upgrade for these type of rentals is slightly higher, as the rental car places have rented out most of their cheapest cars by then, and they are bridging themselves for the afternoon as most rentals come back in the afternoon at or around 5:00pm.