I am going to add in some other considerations like dealer service, insurance (may be similar between vehicles), repair & replacement costs of items like the Michelin PS2 tires. Don't know about trunk capacity but the XF's is decent enough to hold a large cooler and groceries. Plus, in the US we get luggage.

I had a '94 RX-7 which had twin turbos. Don't know if the RX-8 has as well but replacement of that down the road could be another consideration.

Finally, I believe the XF is probably going to retain more value because they are rarer.