I have several reasons for disliking the Magnum T-56....
*They are longer, bigger around, and heavier than the TKO 600. The overall length of the TKO 600 is 31". The Magnum T-56 is 33.5". The TKO 600 weighs 105 lbs. The Magnum T-56 weighs 135.
*I normally don't list prices here, but Tremec holds everyone to a minimum advertised pricing agreement, so pricing should be the same no matter where you look. The price on the TKO 600 is $2450. The price on the Magnum T-56 is $3050.
*The TKO 600 is rated for 600 lb-ft of continuous torque. The Magnum T-56 is rated for 700. I have guys who routinely put more through the TKO 600 than that, by almost 50 lb-ft more. In addition, the TKO can be upgraded by cryogenically treating the gears and shafts along with REM polishing of the components. This combination increases the capacity of the TKO by 15% and still makes the TKO cheaper than the T-56.
*A TKO 600 can easily be used with a mechanical linkage. The Magnum T-56 can be used with a mechanical linkage as well, but the midplate of the transmission has to be removed and machined for clearance and for the addition of a ball pivot. This means the transmission can only be used with passenger-side pivoting forks, which means that the fork must move towards the front of the vehicle in order to engage the clutch. The T-56 is meant to be used with a hydraulic throw out bearing in stock form. Unless there has been some new R&D done in the past year or so, these two ways are the only ways that I know of for clutch actuation with a '56. What's bad here is that if you have an engine that needs the torque capacity of a T-56, then you also will need a twin disc clutch if you want it to be a pleasure to drive. A thicker pressure plate/disc combo exacerbates the clearance issues and necessary modifications.
*Everyone touts the addition of the 6th gear (more is better, right?) of the T-56, but with the careful study of the engine's camshaft, the rearend gear, the tire size, and the available overdrive ratios, you can about put yourself in the same spot with a TKO 600.
*The TKO 600 can be used with a Toploader bellhousing by shortening the input shaft. You really have no hope whatsoever of sticking a T-56 up in the same spot as a Toploader.
*When the Magnum T-56 first came out, they had output shaft seal problems, which meant that we had to send out warranty replacements. Now, to be fair, this isn't an issue now, but just added to the "cons" when these transmissions were new.