Rally... = .15 engine... usually doesn't have that great of a ground clearance.. but it's fun in light dirt and gravel... can fit touring car bodies on it.. if you got a touring car most are 1/10th size..

Buggy = .15 -.21 engine... Has decent ground clearance... and you can do jumps with them.. and you can run them in the light dirt and gravel and maybe a little more bumpy.. Usually 1/8th though.. however there is 1/10th ones out

Hope that helps...
<~ I like stadiums.. has the ground clearance.. and sorta like a rally+Buggy+monster truck..