I just ran my own test just to see what comes of it. Recorded something with both SPDIF and a pair of balanaced jacks. The noise floor is higher with analog, that was expected. Latency was about 1ms faster with analog than SPDIF. So the RME's converts are a tiny bit slower, but shouldn't be noticable. But in regards to sound I can't tell the difference between the two, at all.
Edit: I don't know what kind of SPDIF cable I have, but it's not a super cheap one.
Edit 2: Doing it again with fresh ears, I can tell the difference. Must have something to do with the preamps and the cable.