I have both and for scanning 35mm, the 5000 is king.
The ICE feature is a real time saver and runs MUCH faster than the one that runs off of the 4990.
I only use the 4990 for the odd occasion that I shoot MF on my pinhole or Holga, or when I just need to do document scanning. I've never actually tried to use it for 35mm scanning.
If you get to the point where you're considering going with the CoolScan V to save some money, don't do it. The amount of time that you'll save with the faster 5000 makes it well worth the money, even if you're just scanning 1 or 2 rolls.
I rented a 9000 to scan a bunch of negatives from my wedding (before I had the 5000 and when I was in my DSLR phase), and it was great. Much less grain accentuation. But, it's big, more expensive, and the film carriers are kind of a pain in the ass compared the the motorized feed on the V/5000.
The only drag with the 5000 is that when you feed a strip in, the leading edge of the first frame doesn't lay flat so you lose some image quality on that part. It's never ruined any photos for me, though. The solution for that is to use the no longer produced FH-3 strip carrier that you then feed into the supplied single slide adapter. They'll come up on the auction site every once in a while and usually go for around $100 (way over what they sold for originally). I never bothered with one myself.
Randy, you thought you were a rambler???