Well, the way I see it is that if you get a larger disc with a slower rpm, and a smaller one with a faster rpm, they may well have the same tip speed but at lower rpm ranges the disc will get more bite and engage more frequently, if that makes sense?

It all depends on your design though, you want to maximise bit as much as you can - even though 720's drum spins at about 15k and is only about 100mm diameter, it has a single tooth design and well over 10mm of bite if I recall. That plus the speed it runs into other machines at means it can get monstrously large hits. (Again, all what I've heard, apologies if any of that is wrong!)

If your disc ends up pushing 10-12,000 rpm then I'd consider a larger disc to be honest. There's lots of variables though like weight and how much of the mass is towards the outer ring... That's why Archangels bar has those heavy tool steel teeth, it contains more energy at lower RPMs, so may get similar/better performance than one with more equal weight across the bar hit-wise but will also have the perk of getting regular engagements.

Theoretically, I think anyway. The extent of my robot building really goes as far as the computer game, so I'll probably need someone to verify haha.