I've been on startline for UTMB (last year) and TDS (this year), DNF both for different reasons, having run long sections of the two all I'll say is both are tough, but in my experiance, mile-for-mile the TDS is tougher, bit more ascent descemt, more technical, rougher, rockier, muddier ground (closer to Lakeland 100 or lake distriuct rough trails in places). UTMB trail is generally pretty runnable stuff, its just the extra ascent/descent picked up over the extra distance that makes it the "daddy"!
My point in this comparison being. Maybe don't treat the TDS as a step up to UTMB, best to try CCC first unless your hugely experianced with mountainous stuff elsewhere. Though 35 or so miles shorter than UTMB and only 10-15 longer than CCC (based on full routes run - which we know isn't always the case with these events) the extra difficulty of the course made it feel more like a 100 miler than a 50. Having done the Lakeland 50 this year very comfortably the TDS (70m~) was very much a different ballpark.
Knowing what I know now, I'd have tried CCC first and then either UTMB or TDS the next year. But what the TDS does have going for it is that its the only single-runner UTMB series event which didn't go to ballot last year, so if your not hung up on a 100 and just fancy a really serious challenge, this may be worth a punt.