If you are doing an IR after your ME/CPL then it is quite useful in terms of pratice for the Assymetric Circuits on the IR. I personally didn't like the sound of the single engine CPL, sounded harder than the Multi engine one with all the PFL's, emergencies etc.
You get longer legs on nav and diversion as the aircraft is faster, but only about 20kts and the Seneca certainly is far easier to nav with, as you have a fantastic view out of the window and cruises 2 degrees nose down.
All in all, I would say they are similar. Having two engines isn't really that much harder as you are always moving the wiggly bits at the same time as each other and Assymetric, as long as you are disciplined, I would say is easier than PFL's and all the emergencies thrown at you.