Hi REV,
I've ridden both models but I have only owned the 400/4 and that was more by accident than design. Both models are very popular and have a good back up in parts and knowledge. The only fly in the ointment that I can see is that the 400/4 is physically a bit small. I'm 5' 10" and I felt too big for it, I didn't feel part of the bike and was perched on it. The XS, on the other hand, was a proper bike, size wise. I'm sure if I had been 2 or 3 inches shorter (and at least 2 stones lighter!) the 400/4 would have been perfect for me and the XS too big.
You might find the 400/4 a bit buzzy and the XS a bit lumpy but both are very good bikes. If you can get a test ride on both of them at least you'll know which suits your style and size.
Now I've read this through I'm not sure how much help this has been to you.