There really isn't a "better", just what works best for your situation and budget. You can and should talk to both architects and design/build firms, also general contractors to get estimates. The first two are going to be more expensive but will also allow you to be less involved. Good luck!

We want to bump up over our kitchen to add another bedroom upstairs. At the same time we'd like to see if we can knock out the wall between our kitchen and dining room.

My DH thinks we need to call an architect to draw up some plans but can't we go directly to a design and build firm? Which is the better route to go?