I've owned over ten sebenzas in S30V, and six or seven in S35VN, and the biggest difference I can notice, is that the newer S35VN seems to take a much sharper edge much much faster and easier on my sharpmaker ceramic rods. I've used em until they're damn near dull, and within a few minutes of applying light even strokes, the blade will treetop hairs off of my arms with amazing ease. This isn't all in my head either, its tried and true, repeatedly.
Watch out though, if you do decide that you want to try out the newer S35VN, and I highly recommend it, not all Sebenza 21's have the newer steel. You will just have to check the birthcard, but most of them born in 2011 have it, maybe as early as January, but I know that they were using it in February. Not all the newer insingos and tantos have it though. I recently saw an Insingo Sebenza born in may of 2011, that was still made using the older S30V steel. Another way to tell, if you don't have the birth-card, is to look on the tang of the blade. The S35VN blades are marked with a "V" where as the S30V blades are marked with a "S". You would probably have to have the knife disassembled to see it though.
Hope this helps