Depends on the condition of both. Lets just say they are both in fair condition, as an example. Then I would say, if the Slash 4x4 is RTR, you come out winning in the trade, by far. Even with that Chassis conversion done to your stampede, unless you are including a nice servo and/or transmitter, it still has less value than a fair condition, RTR Sl4sh would have.
Hard to really give a good opinion without pictures/descriptions.