Usually stay with dual chamber pipes, you will see as they have like an inner wall (chamber) insde the pipe. It usually can be seen by looking thru the exit tube of the pipe. The ones that are cheap are the single chambers, those give plenty of torque but not much top end. The single chamber are sold most of the time cheap on auction sites.

The length of the pipe also affects its performance, on-road engines use shorter pipes (more top end).

Sometimes you got to spend the money trying, testing different kinds. I rarely buy them new, get them second hand on auction websites and if I dont like it resell or for bashing back-up pipe. Even the material they are made of affects performance, back pressure has an important role on nitro engines.