[quote=TentacleLord]
Tanks may be faster on flat terrain and usually better armored, but mecha are more mobile and quite usually better armed. Add that to the fact that the Junkyard is very often flat terrain, which makes the mecha a much more versatile choice. Note, this does not mean that conventional weapons like tanks are unused, and that's why if you wish to use one, you should be very aware that you can be outmaneuvered by people using said mecha. If you still wish to do as such, I would suggest scaling it up in order to compensate for such things.
[/quote]

Judging from this, it sounds like the junkyard is comprised of literal mountains of trash. Uneven terrain doesn't deter armored vehicles as much as people think. If the junkyard is an expansive landscape of small mounds of junk, petite hills and potholes, tanks and light vehicles (like APCs, IFVs and mobile gun systems such as the M1128) aren't going to have much issue with such a landscape. If the tank's weight is an issue (which it shouldn't be, considering that machines weighing in excess of 50 tons are present in the world), then the tracks can be widened to decrease the vehicle's overall ground pressure at the expense of overall speed. 

Mountains are going to be a hindrance to anything that tries to maneuver through them, though walking machines will have the capacity to climb these hills far better than a wheeled or tracked vehicle can (but only to a certain extent, as their overall mass coupled with their poor center of gravity will inevitability cause them to tumble backwards should they scale the mountain too far---unless of course they have arms or something, or if they possess four or more legs and are lower to the ground). 

This is all technical stuff, and probably shouldn't be brought to the table since this RP appears to be more about fun rather than plausibility.

As Spoo said, it's all about the aesthetics. If I bring physics and actual science into this, it's going to ruin the party for everyone---including myself lol.