Just had a few minutes to spare to A/B them. If you want a budget DAC to use until you upgrade then get the UD, no question. The difference in SQ is very, very slight - unlike the difference in price.
The ODAC has three advantages for me:
1) It runs much cooler so I'd expect it to last longer - though the UD doesn't get as hot as I thought it would. And, for my Epiphany ODAC at least, the ODAC feels more substantial, which also suggests more durability.
2) I mostly use my laptop as the source, and I quite often use my laptop, well, on my lap. That means I like a couple of feet of cable between my computer and the headphone amp. It's easier to keep that cable in the digital domain with the ODAC. I'd need a USB extension cable to do that with the UD. This also means that the cable that gets plugged/unplugged all the time is a more disposable standard USB one with the ODAC rather than the DAC itself with the UD (though that can also be addressed with a USB extension cable).
3) It works with my phone (Intl Galaxy S3) without any special software. This is not a huge one for me ATM, but it could be if my working habits change.
The UD has one advantage - it's smaller and lighter so is more portable.
Unless you can spend some time with both devices, I would choose based on the above rather than the SQ. If pushed, I'd say the ODAC was a little more detailed, and the UD was a little more musical - but I'm also pretty sure that I wouldn't be able to definitively pick one over the other in blind testing. But you should also know that it's still early days for me in the world of audiophilia, so if someone with more experience than me says there is more difference than I can hear, believe them.