After looking at both, the biggest differences I noticed:
The 23S has the option for the SSQA front attachments. I enjoyed this on my L, but the reduced lifting capacity for this option could be an issue.
The 23S has an option for a roll-over mid-mount mower, the 25D does not.
The 23S has a prettier dashboard, and the 25D has been known to leak water in the dash, causing issues.
The newer -1 25D models have the updated seat with arm rests, reclining, etc, and I didn't notice any difference sitting in the two.
The 23S has a new K-Connect system for snow equipment vs the tried and true BX2751 subframe. Seems that some have issues with it.
The 23S can disconnect/remove the loader without having to get off the tractor.
None of these upgrades on the 23S mattered to me except the SSQA, so I opted to go with a low-hour BX25D-1 that I found a screaming deal on. I'll figure out a mount up front in the future, but so far, I'm only missing it for forks and don't plan on using a grapple or other equipment up front.