edit: christ I didn't know I wrote this much! Sorry! Overall tl:dr is wedge design and traction + desired display of power = more important than theoretically more powerful drives.

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It depends on lots of things, grunt of the motor(s), reduction from that motor, wheel size, tyre material, floor surface, what you actually want from the robot, etc.

Mathematically, a pair of speed 900s or similar have double or even triple the power of four drills, at least in theory. But then if you gear the drills to run a robot at 7mph, and the speed 900 drive means you have a theoretical top speed of 27mph, then the drills will likely defeat the other robot in pushing power (assuming no losses in traction, so still only in theory).

So, assuming perfect grip (no wheel spin at all, which is very very unrealistic), then if geared for the same top speed, a tug of war between a pair of s900s or similar and 4 drills will result in the s900 bot winning every time. But if both robots are powerful enough (and even 2 drills are) to spin the wheels when trying to push something, which is how it is in real life, then if the machines have the same wheel size/tyre compound then neither with push/pull the other significantly or impressively in a tug of war.

tl;dr listen to Craig he's right.

It depends also on what you want from the robot. A machine like Jarvis' Ricochet pushes everything but is quite slow, and whilst Satanix might not out-push Ricochet on paper, if it gets under it, the speeds that Satanix can reach will result in what appears to be a much more powerful pushbot.

If you just want to push others then drills will do the job. If you want to have an impressive rambot then you need speed, and you don't see very speedy and also pushy drill powered robots because drills aren't generally powerful/reliable enough beyond about 10-12mph.

All theories and shoulds become irrelevant if you can't get under the opponent. Design of the robot, and its wedge/scoop abilities, are possibly most important of all. Two drills in a shallow wedge robot will control a robot powered by 50 speed 900s if the s900 bot is badly designed!