As you say, I think using treads would provide the mech more stability at the expense of speed. Specifically, the mech should receive bonuses to resisting things like bull rushes and trip attacks...maybe it should even be immune to trip attacks. The downside would be perhaps it only moves at half the speed it would otherwise, and cannot trample.
i agree with you there on most points, EXCEPT for the trampling part. wwi and wwii have some pretty mean examples of tanks doing bad things to people. in fact, trampling with treads should probably be more a factor of normal movement than an actual attack action.
also, treads make it pretty easy to turn, so perhaps increasing the maneuverability for purposes of turning, and reversing would be apropriate for treaded mechs?
Treads should overall be worse than legs, I'd think, or else everyone would be using them instead of legs (and we'd have a world full of tanks instead of mechs). I'd say the systems are probably simpler than legs too, so make them a bit cheaper and perhaps even lower the mechcraft DC. But again, I think in most cases the disadvantages should outweigh the advantages.
ok let's see. here's the laundry list of ideas so far.
1) bonus vs bull rish and trips
2) decrease in speed
3) no ability to jump or climb regardless of maneuverability
4) no ability to step over obstacles whatsoever
5) easier to turn and put it in reverse
6) easier to pilot across rough terrain, assuming you dont bog down in tall obstacles (certain obstacles should be harder for treads to overcome.)
anything else ot add. i plan on working up a mechanic or two for them and i'll post it here for your perusal when im done.