I also have the XOno. Someone told me that the XP-25 is basically designed for those who have/need two phono inputs because they have two tables or two arms. It's also for those who like the convenience of front panel dials for easier loading and gain changes. The internal dip switches of the XOno are inconvenient at best. However, there are many more options. I load my Supreme at 84 ohms. That setting is not an option on the XP-25, but it is on the XP-15.
The XP-15 does have the same outrageous number of loading options that the XOno has, and they are even on the back panel so you don't have to remove the top plate for changes.
I don't know how the three compare sonically as I've never had the XP series in my system. It may be time to call Reno HiFi to do a demo for the cost of shipping. It's the only way to know for sure how it sounds in one's system and if it has the right loading settings for your particular cartridge/wiring combination.