I would rather have the 48" vanity. Make sure your walls are square and plumb! Measure in multiple places to ensure it is the same width and depth everywhere. Or if you can get a 46 or 47 inch vanity, then you could just add a thin filler on each side so its not quite the worry of an exact fit for the vanity. Then get a custom template fitted top for it.

With a 48", you get more storage and counter space. It doesn't have to look like a plain box. You can dress it up with feet or a valance bottom to give it more of a furniture look.

Transitional Bathroom · More Info

Those inset drawers....sigh...

That is beautiful.

If it's custom, they can build the carcass but not install the face frame on the carcass. Or they can install all of the face frame except for the stiles. The stiles can be cut say, 1/4" too wide.

Once on site, the carcass is installed, then the stiles can be scribed to fit the side walls. Then the face frame installed.

We're doing a custom vanity because I want it to go wall to wall, plus get max real estate on top. It will also sit between 2 walls in an alcove.