I picked up my Fatboy last night after not using it for a while. I thought I'd just use the same settings as I do with the Slim...5 for a first pass up to 7 for the last two passes. I promptly gave myself three nice weepers on my upper lip. The Fatboy is much more aggressive than the Slim set on the same settings. I got a really good shave, but not without some trauma. I found myself having to use extreme caution on 7 where I don't have to be as careful with the Slim. I really felt that blade.
Overall, I find the Slim to be a much more elegant razor, visually and performance-wise. If the Slim had only been produced for a small number of years like the Fatboy, this would be the razor everyone is clamoring for on ebay. I always admire its ingenuity and looks when I use it.