I think they are different enough to warrant having one of each.
The ESQ-1 is one hell of an analog/digital hybrid, and maybe the closest synth relative to a SID synth (without actually being a SID synth). Three oscillators. Analog filters. Three very flexible LFOs (if I'm not mistaken). 80s digital synth sounds with a warm curtis filter. Yummy.
I've never used an SD-1, but on paper, it looks to do a different type of digital sound. To my ear, the SD-1 sounds like a very 90s rompler. Nothing wrong with that sound, but not my flavor. If you want to dig into synth-programming territory, I imagine you'd find some very grainy, cold, glassy sounds, which would also be awesome.
So there you have it. My somewhat biased opinion towards the ESQ-1 and the SD-1. Happy synthing.