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I'd say neither adds a lot of stiffness, but the floor mounted bar certainly adds less.
If you imagine the chassis floor by itself isn't stiff- you can imagine it as a piece of paper. Twist the ends and it flexes easily.
Now if you add the roll bar structure to the middle of that paper it will stop that one little area from twisting as much, but since you attach it to the paper with four little points, the paper will mostly just flex around those points, and it certainly doesn't help stop the rest of the sheet twisting.
Next version- bend the edges of your paper up at 90 degree angles, forming a U channel. This is more like the chassis floor and frame rails. Now twist it lengthwise- it's stiffer, but the tops of the paper "frame rails" are free to move in opposite directions to one-another, so it's not exactly stiff. Now if you add your roll bar attached to those vertical U walls, however, they mounting points don't flex the paper, and you're adding a little bit of real stiffness.
Next step- attach the roll bar not just at the bottom, but also at the top corners to the roof. Now you've closed the top of your U channel, and it begins to act more like a tube. Ever try and twist a tube? So attaching the roll bar at the top corners helps a lot, not just in the area where the bar itself is attached, but also the rest of the chassis- that's where you start to see real stiffness gains.
A floor mounted bar also attached at the top is probably stiffer than a sill mounted bar not attached at the top, but I'd go for sill mount every time.
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