Both motors are ok, but if you're gloing to be running it in your MLST Swamper, the the 6800 might be the better choice.
It wil also depended on what you arwe loking to get out of your BL motor. Are you lookiing for wheelie popping torque, or "stupid fast" top end? High speed runs are hindered by the increased weight and not-as-smooth-drivetrain when compared to something like the RC18T. BL motors with the higher KV rating will be tuned for those high sped runs.
For more torque, look at that 6800 Mamba, the Hyperion's 5K or 6K, AON 4900 or even the Tekin motors.
Batteries will always play an important role in how much speed and power is unleashed from your motor(s). Better batteries or more voltage will net better results.