Add a bathroom or a Master Bath?

[Eric Dymesich]

I'm in the process of buying a side by side duplex. It has 2 nice bedrooms on the second floor with closets and a third small room for an office or a baby's room. It has a full bath on the second floor but no bathroom on the main floor. It is an old house with a big dining room and living room. Off the kitchen, there is a 15x14' room that is not conditioned. I'm going to finish that and open it to the kitchen it as the dining room/kitchen open concept. Then turn the current living room into a master bedroom. Also, I'm going to add a bathroom on the main floor. The question is: should I make it a master bedroom/bath or should I keep the bathroom separate? 

Thanks!