I always prefer directories that offer permanent listings. I won't mind paying $XXX for a permanent listing, but I don't like the idea of paying every year to maintain backlinks for my sites.
Both Permanent and Yearly options have their own advantages and disadvantages. Permanent listing is better for directories that are under PR4 and for directories above PR5 they should have both options!
I agree at PR6 you will probably get more interest in yearly because they are more affordable. PR4 and under (for sure PR3 and under) permanent is the way to go for offering value. I think it is best to offer both options especially for more expensive featured listings.