I own both the Jamman Express XT and the Ditto. I prefer the Jamman XT because it has more features, but the Ditto does have a few advantages:
1. Requires less power. Dedicated power supply users might run into some trouble. The Ditto can be powered from a typical power supply at 100mA. The Digitech requires 150mA, so will take up one of the higher output ports. Not an issue if you're using wall warts or a 1spot type of power supply.
2. The Ditto stores the last recorded loop, even after disconnecting the power. The Digitech doesn't. What this means is that if you typically use the same loop for some reason, you will have to re-record it every time you disconnect power to the Digitech.
3. The Ditto has a slight edge in size, but it's not really that much different.
I had a Ditto, got rid of it for the Jamman Express XT (sort of), and now I have a Ditto again (and no Jamman) if that tells you anything. I posted a lengthy review somewhere on here, but the JMXT was noisy on anythign except isolated power, the soft touch footswitch was too soft and prone to being held too long accidentally, and the bit WWW mentioned about saving the last loop is key, since sometimes I would flesh out a good idea and not have something handy to record, or have to get going and I knew I could just leave it and come back. It sucks that people are having issues with TC stuff, but I've had a half dozen of their pedals without issues, so it's not like they're all bad.
I preferred the JamMan Express XT on paper, so I went with that one (also got it on sale for $79, so that was another deciding factor). But I do dislike losing whatever is stored on there when power is disconnected. Not a huge deal, but I wasn't aware that the Ditto actually held the last loop session in memory.
I have 2 TC pedals, the Dark Matter, and Spark Booster. Both are great, though I am very gentle with my gear. They haven't seen any gigs, only home playing and rehearsal playing. But no problems so far. I'm getting a TC Polytune 2 soon as well.
I say just keep rocking your Ditto until it actually fails you, if it ever does.