They have the same pinouts, but they draw more heater current, and output more power, which would overload some OPTs that are barely big enough for 6550. KT120 are also heavier tube than 6550, so you can't run them sideways in amps with vertical PCBs like old ARs, even though those have enough PS and tranny to handle them. The later AR amps with horizontal PCB like Ref110 can run KT120s. The new AR Ref75 is able to make 75W with single pair of KT120, said to be a real honey of an amp.
Jim McShane recommends the new TungSol 6550s for amps that used SED 6550. He says his customers are happy with them.