FWIW, I'd be keeping the 25XX tanks and getting rid of the 15XX ones for 120.
Some information for Jobo's Newsletter (now long gone), circa 1999:
"So which tank system should you select? It all depends on your particular needs. If you will be processing mostly 35mm film, go with the 1500 series. If you are processing mostly 120/220, either system will work, with a very slight edge in favor of the 2500 series. If you plan to process roll film and sheet film and don't want to spend the extra money for an Expert drum, the 2500 system is the answer."
Also, a larger diameter reel (2500) means that there are fewer "layers" of film within the reel. This will mean a more even coverage over these layers. And, as the angle of attack of the film into the liquid is less on the 25xx reels as well as less layers, you'll get a more consistent and less aggressive immersion over more of the film for longer.
You'll also have the option of developing sheet film with the 25xx tanks. I develop 7x17" sheets in them.