Its a tough call between the two each has its pluses in areas as clearly stated already. If it was me I would go with the Z06. IMO the Z06 is a better all around car than the Cobra is.
I don't think you can go wrong with either car. Also look at what is used and pick up the best overall deal.
Is she going to drive it year round? All will be a big change from the VW in winter.
I'm not a big fan of the RX8 or the IS 300. I would take the G35 over the Caddy based on looks.