The CM Jets tank is a better design than the stock tank. It is shorter and wider and places more fuel over the CG. Therefore there is less of a shift of the CG during flight. If that sorta of thing is important to you then the CM Jets tank is a worthwhile investment. However, the Rebel isn't that sensitive about CG and folks running the stock tank don't complain about it.
My small Wizard had a horrible tank layout with all the fuel in front of the CG. The CG would shift 1" during flight. For that small aircraft it was too much. Early in the flight much up trim was required and it changed as the fuel was consumed. I re-engineered the model and now the fuel is centered on the CG. Much better now.