From what I have read, is there is not a nickles worth of difference in HP between the two. Lower-units and CARB Ratings (3 star vs 2-Star for the XS) are the main differences.
taken from another website:
Enhanced 3.0L OptiMax Bass Engine Offering
Mercury Marine is proud to announce an enhanced range of 3.0L OptiMax bass models designed to provide even more propulsion options to maximize the performance of your boats.
Joining the popular Pro XS series will be two new XS models that feature the Torque Master gearcase and two new Sport XS models with the Sport Master gearcase. These introductions round out a strategy to match the OptiMax brand (Pro XS, XS, and Sport XS) with a specific gearcase that better targets the performance needs of each segment within the bass market. A breakdown of the new 225-250 horsepower bass offering follows below.
1-225P73ED 225L Pro XS Verado Gearcase 1.75:1 3 Years Standard Production
1-250P73ED 250L Pro XS Verado Gearcase 1.75:1 3 Years Standard Production
1-192647GHD 225L XS* Torque Master 1.75:1 3 Years Racing Production
1-192847GHD 250L XS* Torque Master 1.75:1 3 Years Racing Production
1-192747BHH 225L Sport XS* Sport Master 1.62:1 2 Years Racing Production
1-192947BHH 250L Sport XS* Sport Master 1.62:1 2 Years Racing Production
1-192947DHH 250L Sport XS* Sport Master 1.75:1 2 Years Racing Production
*Built to order.
The biggest changes are the addition of the 225 & 250 "Sport XS" models, and the 225/250 XS models with the Torquemaster Gearcase (and 3 year warranty).