I really really really want to suggest the RX-8, because you deserve the fun car as the bonus, but it really doesn't sound like it will work for your needs. Yes, you can buy a set of winter shoes for it and that will help, but your winters and ground clearance sounds like an issue. If you really could keep the RX as a weekend car (or get a car cover and store for the winter), that would be perfect but it just isn't practical enough for even a 1/4 Costco run (says the guy who has done home depot runs in a FR-S with lumber sticking out the back).
In your pro/con list, write down how many times in the past 6 months that the RX-8 would not have worked. If you have less than 3, then you probably could get away with it. If more, you probably would regret it.
I know your buddy Steve would say "keep the 5". :)