I have a SST1200es in house and I absolutely love it, I haven’t heard of the MarkOne either but if I were to get another printer it would either be a used 1200es or possibly a Fortus, I would be staying with Dimension for sure. I’m a firm believer in both brand loyalty and sticking with what works, and the Dimension has never let us down. It has its issues like any other printer would, but the quality and strength that I get from the prints is hard to beat. We primarily use ours for form/fit/function prototyping and as foundry patterns and the hobby printers can’t touch the dimension for those kinds of parts.
The soluble support material is in a league of its own as well, there’s essentially no limit to the geometry you can print. I built a wash tank myself instead of purchasing one using a couple silicone pad heaters, a controller, and a cheap parts washer tank. It may not work as efficiently as one you can buy for a couple thousand bucks, but it does the job and it was cheap to build. It dissolves the support material right out. Perfect for foundry patterns where you need an untouched, perfect model surface to cast from.
I don’t know if that helps your decision, but I do like my 1200es a lot!