I have had both (really, all three - the Mkii for a while as well). Currently I have the Mkiii version.
I have always acquired them used (and saved quite a bit from new) - and while still more costly than other monosynths out there, I really do enjoy the sounds that come out of the Doepfer unit (any of them) and for me the DE has a place as a no presets, compact, all knobs visible, type synthsezier.
Having said that, I’m happy with the Mkiii at this point - in particular given the multimode filter. You probably know that the internals are different from Mki to Mkiii as well, and with the filter variability I’m very happy with what I am able to get out of it vs. the Mki. And as was also pointed out - no warm up time needed (like the Mkii). The only thing I miss vs. the Mki is there were a couple of patch points on the Mk1 at the bottom (for LFO1 and ADSR IIRC) that changed and I probably preferred what the Mki offered vs. the Mkiii (apparently this can be tweaked via the insides of the Mkiii, but not a big enough deal for me to try it).
Anyway - if I had a Mki and was completely happy with it, would I buy a Mkiii? Probably not - would go for something totally different. But if I was deciding between the two - I think I would go with the latest version.
Lastly - I’m a DE fan - great little boxes.