I drive a 2015 SRW 3500 Cummins as a DD. Difference is the 2500 has the coil set up and the 3500 still uses leaf springs. I like it just fine and so far zero problems. Biggest thing is tire psi. Spring and summer when we are traveling and towing a lot I run 60 front and 75 rear. Unloaded it rides fine to me but can get sorta rough on dirt roads and such, but very compliant IMO. Off season I run 60 front 55 rear and its great. Best part is the 3500 has the tpms but doesn't keep a stupid light in my face when I set psi where I want it. That was a big plus to me. When we were looking in 2015 I drove both the 25 and 3500 and honesty could not tell one bit of difference when tire psi were comparable. For the higher payload, higher RAWR, no TPMS alarm/light, it was a no brainer for me. My particular truck has a payload capacity of 3890 pounds with the Cummins. For pulling a 5th wheel that worked out much better than the 2500 that was 2400 and some change. I'm definitely of the mindset that for the very minimal price difference between the two that the 3500 makes sense if your gonna be using the truck to tow.
If I missed anything let me know. I'll try to help best I can.