Overall, the RX 8, however from the practical point of view the Solstice is a better choice. The reason being that the RX 8 can be very unforgiving on a hot start, has to be warmed up before turning it off and needs to have all fluids checked at least weekly. When owners forget these rules, repairs follow (and they are not cheap in most cases).
So the bottom line is for a driver who just wants a car to get in and go, the Solstice is a better choice.