That's quite the dilemma! Although this won't help much, both are truly excellent cars and I don't think you'd regret either. The MX-5 obviously has the advantage of open-top motoring and, as a rear-wheel-drive sports car, is perhaps a little more involving to drive. There's also a great ownership network with the MX-5, too, so you can join in with organised owners' club events or easily modify it to make it faster/more enjoyable. That said, I reckon my money would go on the GR Yaris. It feels very special and I'm sure will hold its value well. If it's your only car, don't underestimate the extra practicality of the Yaris, either.