it depend on your track.

the mbx-5 is a better handling buggy around a corner and on a smooth track. as where the k3 will beat it over ruff rutty ground and big jumps.

if your track is smooth with not to many jumps then i would choose the mbx-5 but if not go for the k3.

p.s if you don't want to put that much into a buggy go the mbx-5 on ruff stuff the mbx-5 is not going to loose in a club meet race only at nationals levels.

it is all up to you

decide if your track is ruff or smooth then go from there.