SDW has a real sense (at least to me) of journey. You set off from Winchester, and gradually the countryside changes as you head East, the hills getting bigger and steeper (possibly a bit to do with legs getting worn out), passing through villages, glimpsing the sea, then finally finishing that last long brutal climb and then down into Eastbourne.
K100 in contrast feels a bit more artificial, if that makes sense. There’s a lot more gravelly fireroad and the K100 hills are smaller. There are some nice bits of single track but you don’t get the same sense of having traveled (but then that’s not their intention of course).
That probably makes no sense I’m afraid. You’ll just have to go and ride them yourself. Which you won’t regret (either of them).