The difference on track between the two cars if set up perfectly is likely to be very small. However the difference between a badly set up car and one set up well can be seconds per lap.
So... go with the car that you can get setups for (from good drivers), for the tracks and tyres you race on, as that's where most performance will come from. Setups don't transfer well between tyre makes, surfaces and grip levels, so you need setups off the fast locals, not random setups off the internet. So, what do the fast locals run where you race?
The Xray is probably the stronger of the two cars (and is probably the strongest car period), so if you race indoors with hard barriers that's also something to consider.