Easier to control. For instance, you can easily just dribble the gas into the tube. I also feel that if you have to nurse a tube home - say a leak that your sealant didn't' get, you can seal it up better to keep the remaining gas in the cartridge long enough to get home.

Also, most of the knob ones (Planet Bike, Portland Design, and my favorite - Silca) tend to be the smallest and lightest. Small is good.

J.