Synth is a pretty broad term jpapas. It really depends on what you're going to do. When I think of a "synth", neither of these types of boards are the first thing to come to mind for me. Boards like a Nord lead, Prophet 5, Moog Voyager, V-synth.... these are what I think of. Still, a sample based workstation is definitely a type of synth.

Really, In my way of thinking, the short answer is if you already have an Es and a regular Mo, and you don't need the phrase factory functions and arps, the mlan setup, or access for plug ins like the AN150 or PLG piano board, etc, then something like a Tyros II could probably suitably replace one or both of your boards for what you want to do.

I didn't mention the sampling functions on the Motif series because to me they are hardly worth the bother vs inexpensive and much more intuitive software samplers.

If you're looking for a synth to do primarily analogue ( or an modeled ) stuff or cutting edge synth voices, then neither an arranger or a workstation would be the best choices.

AJ