Adding to Peter's response, it would depend on your budget, whether the XR18 was meeting your needs with respect to no. of inputs/outputs/buses/DCA groups/FX units/routing/etc.
My band runs a Behringer X32 with an additional SD8. The new WING solves so many limitations we run into with the X32, but nothing is a deal breaker so we can't justify the upgrade cost.... for now... until then we just drool over the specs page ;-)