Originally Posted by Head2Wind
it sits lower, transitions are quicker, front reacts quicker, more fun on road because it opens up the tire selection to nice sticky stuff..... AND if you want you can also mount Heidenau K60 Scouts 17/19 for DS work
Have to give a big +1 on all this. I'll also add that the bike feels more natural (for lack of a better word) on pavement- making fast riding an easier and more enjoyable experience (sorry for getting all Zen). It's like the bike was made for a 19/17 setup.
If you decide to do it, bite the financial bullet and get everything you need for the 2nd set so it's a quick and easy switchover if you think you'll be wanting to keep the 21/18 for more technical riding.