I would pick the Constellation's itinerary as it is more interesting to me. There are more port days so I don't need the latest and greatest on the ship as I am going to spend the limited sea days relaxing in a chair watching the sea go by.
I haven't been on either of these specific ships, but have been on several others within both classes.
The S-class ships like Equinox have more and better specialty dining, the Lawn Club area, plus the Equinox has the Hot Glass Studio. The buffet area is set up in stations which seems to help with traffic flow. The layout of the Persian Garden is much nicer with the heated loungers facing windows looking out over the ocean, instead of in an inside space on the M class. It also has 2 aft-facing bars, so if you like sitting and watching the wake with a drink, the S class might be for you.
The M-class like Constellation has fewer passengers. I like that the pool is on the same level as the buffet- makes it easier for me on sea days. I love the martini bar on the M-class- somehow never as crowded as the one on the S-class, even while the S-class one is larger.
Constellation is scheduled for drydock in Spring 2024, but is over 20 years old (built 2002). Equinox is also scheduled for drydock in Spring 2024 and was built in 2009, so 7 years newer.