Worth the $2000 difference from a Xeen to you?
I think so - they are both very good for the money. The both have large focus throws, but the travel on the CN-E's is much smoother and a bit heavier. The Xeen's are very light - so if your doing lots of shallow depth of field work with focus pulls - you do benefit.
However I work at a University and the Xeen's are great for the 1st and 2nd years where their focus pulling is less likely to be smooth and I mean't I could buy more lens's for my budget.
Best to try them. Canon lent me a couple of lens's to test prior to my purchase - it sealed the deal for me, I didn't want to give them back
On an undergrad film programme the lens's do get a lot of hammer. 3 years in we have only had to get one lens serviced and that was because it was dropped. The jury is still out on the Xeens, we've only had them 9 months. For use build quality and serviceability are just as important as optical performance. 2 sets of CNE's represent quite a big investment for us - so I need them to last.
Zooms are the next thing on my list to get, as we only have a mixed collection of DLSR zooms - been interested in the Sigma's but not had time to test them yet.
The fact a lot of the lenses cover FF is also great. Our CNE's look great on super 35, but on larger sensors, I assume they'd look better and the CA would seem smaller.