The newer version is F3.5, but has a price tag to go with it as you say, from reviews I've read - mixed responses, yes it's very nice, but the F4-5.6 is very good.
I've the F4-5.6 and am very pleased with the results, when I got mine, I couldn't justify the extra for the Canon as it doesn't get a huge amount of use.

Browsing the bay I notice the there are two versions of this lens, f3.5 and 4.
There's a marked difference in price too. Is the f3.5 so much better than the other?