Krakenup and Apache,
You're both confused. Fortunately for you, I'm a physicist, and I'm here to set you straight.
Both of your equations are correct, but you are both making bad assumptions.
If you use the equation that has only mass and radius, then you have to keep in mind that you only use the mass INSIDE the radius. Jupiter's solid surface, if it has one, is obviously way below the gaseous surface, so you would not use the total mass of Jupiter for your M. (It would be way too high.)
On the other hand, if you use the equation that has only density and radius, then you have to keep in mind that you have to use the AVERAGE density INSIDE the radius. Since the density increases as you get closer to Jupiter's core, you clearly should not simply use Jupiter's overall average density. (It would be way too low.)
But, of course, all this talk is silly anyway, because the heat and pressure at Jupiter's solid surface (if it has one) would be enormous. Nothing could live there. Any life would have to be in the atmosphere. At what radius? Who can say?