A lock off is a one bedroom and studio combined that can be locked in the middle. A Studio has one bed and a sleeper sofa. The one bedroom has a real bedroom, the kitchen, and a sofa that pulls out into a sleeper.
A 2 bedroom is just that, a true 2 bedroom. Depending on you're travel party one might be preferred over the other. When traveling with 2 families, a lock off is nice because you get that element of privacy and the kitchenette.
This year we opted for a dedicated 2 bedroom so no one has to sleep on a sleeper sofa. We'll have one master bedroom, a bedroom with two queen beds, and a full kitchen.

Plus the living area where the sofa will remain a sofa.