200e

Regarding the comparison. Clearly they are aimed at different sequencing approaches and tasks so they don't directly compare feature for feature as say the discontinued 249e and 250e might be able to.  

As mentioned the 251e has several much longer independent sequences that you can clock, loop, edit  reset, etc. in a compact format. So it's 4 long chains with tools to clock in relation or not. 

While the 250e has lots of knobby user interface to manipulate several locked together 16 step channels in a large number of fairly interactive ways. Step length can be controlled and there are many conditional options. 

I'm sure it's a call as to what suits you, differences are enough that I'd want to get both eventually. Right now I have the 250e. I found it not intuitive but once I spent some time it made sense ( and i really appreciate it being in a system with patch storage). My general opinion of the older 249e was you had two 24 step sequences that are separate with less of a user interface. It was more of a program it in sort of thing than a start it up and tweak while running. Not that you couldn't but I felt you needed more of a plan to use it.