Geno:
As I recall, the MTH racks were designed to operate on 27" radius (O-54) curves and are shortened to fit. The early ones have the trucks mounted in the wrong place (too far out), which can be corrected by modifying the car.
The Lionel racks are designed for 36" radius (O-72) and are full-length. They use an "adaptive shank" coupler which stretches out in curves for clearance and retracts on straight track for the better appearance of close-coupling. It's actually pretty clever.
In either case, they don't really start looking right until you go beyond 36" radius (O-72) like around 48" radius (O-96) or larger. A good formula to use is multiplying the bolster-to-bolster distance by 3 for the bare minimum the car can handle (with body-mounted couplers) or by 4 for when it starts looking right.
As for selling off the MTH racks, hard call. I have about a dozen of them which I'm converting over to Kadees. I probably won't buy the Lionel racks, instead focusing on the 86-foot excess-height box cars I have a weakness for.