Apologies if you have already considered this, but with a shelf vanity, if the sink does not sit on top of the shelf, then the sink will start at the counter, and go below the shelf. Its a design consideration (your right hand photo shows it ) -- what do you want visible -- plumbing wise. The depth of the vanity shelf front may need to be adjusted so that the bottom of the sink and the plumbing does not show, or plumbing is selected that is attractive. The cottage-y example has a deeper front on it to hide the sink bottoms. Benjesbride has an excellent point about keeping stuff on shelves clean -- it will take a lot of cleaning to not continually collect dust (it'll show on the shelving, and be embedded in the towels...dust, splashes, etc.). Good luck!

IKEA is your solution. They specialize in pieces that will fit in tiny spaces (think European apartments). They also have special sink tops that fit on their vanities, all at a very reasonable price. Obviously, it's cheaper if you can go there yourself, but they do deliver too. You can get the 24" solid wood Hemnes with a sink top for $250. It has built-in, very useful, storage drawers that hold a ton. We have one of these in our basement bathroom and just installed three of the larger ones in other bathrooms. Drawers are much more useful than just two doors that open; I know because I've had both.

Ikea vanity looks really nice. Thanks for sharing. We looked at vanities from Wayfair, AllModern, Ikea, etc., but opted to work with a local woodworker. It felt good to support a neighborhood artisan. When we come up for air we'll have him craft built-ins and purchased add'l Walnut with that in mind. At this point the Brio valves are roughed in for the shower and wall mount lav faucet so there's no turning back. To clarify, my question is when pairing an undermount sink with a wall mounted faucet and 24x20 countertop for use in a master bath - what's the most functional shape or is there no discernible difference?

In my MB I have a 60” vanity with 2 sinks but I did go with slightly smaller sinks in order to get a couple inches more countertop. One thing to think about besides counter top space is storage space in the vanity. I have a storage closet in my MB so I didnt need to be concerned with vanity storage. A second sink will reduce the amount of storage because underside of the sink and the plumbing. With one sink you could have a 30” sink base and a 30” inch drawer base with 3 or 4 drawers. I would suggest doing 2 deep drawers and one shallow like I did in my hall bath