They're 48k/24bit FLACs, so they are lossless, straight out of Rob's mixing desk/DAW. The highest quality on the DVDs is 48k DTS compression, which is actually pretty good but it's still lossy, so the downloads are of higher quality.
Whether you can actually hear the difference is depends on a whole load of factors but most importantly your ears! That's not meant to be a flippant comment; some people say they can tell, others say they can't. There's no right or wrong opinion so I would suggest if you're happy with the DVDs they're not essential.
If you have Sanctuary II download the two free Sanctuary II demo files and do a comparison.