TinkTatoo said:
...which one is preferable.
That probably depends on your situation.
Booking a 2B lockoff is basically the same as booking a 1B and a studio separately. But the 2B costs a few points less per night than booking separately. So, let's assume your immediate family will be staying in the 1B portion of the room and you'd have other relatives or friends of the family staying on the studio side. If you want to share the trip together, and yet maintain some degree of privacy, the lockoff may work better.
The studio side of the lockoff has its own main entrance off of the resort hallway. In a lockoff, you just have one entry which places you in the living area of the 2B.
In a lockoff, you'll have a kitchenette in the studio side (toaster, microwave, sink, coffee maker). The dedicated 2B will have more closet space in this area.
In a lockoff, you are guaranteed to have a balcony for the studio side. In a dedicated, it will vary by resort, but many do not have a balcony.
In a lockoff, the studio side has a separate thermostat. In a dedicated 2B, there is just one thermostat for the entire unit.
And others covered the situation with the beds. If you'll just have one or two people (who would share a bed) staying in the studio room of a 2B, they may prefer the sofa and extra floor space that a lockoff affords, rather than another full bed that will go unused.
It just depends on how you'll be using the room and what best fits your situation.