There should, in theory, be no difference between jumping with the original vs. stock launcher as long as you angle yourself to accommodate the different in firing position. I believe that the tf2 soldier model (and other class models) do not have perfectly symmetrical boxes that interact with the rocket explosions though, so there may actually be very slight differences in how the two weapons affect jumping.
I don't think this would be enough of a difference to make any jumps possible or impossible with one and not the other though.