Clamshell is definitely easy to keep clean but IMO it is somewhat outdated, and I have not seen it anywhere but in 70s-kind of dated houses.
Clamshell = Ranch? We have ranch baseboards in our 50’s home.
I think even if I lived in a 1970s home for which clamshell moldings are authentic, I would have to rip them out! Can’t stand them.
From reading JLC threads, since we’re building:
A source that was considered authoritative, ‘Get Your House Right’, suggests 8" for 11’ ceilings, 6 3/4’ for 10’, and 5 1/2 for 9’.
Personally, I hate the 3 1/2" clam shell and doubt there’s a ceiling height that could make it appealing. Craftsman proportions generally call for something around an 8" base, maybe angled at the top, but I’m not sure what ceiling height was common for that style.
Whatever fits what you like would be a good choice, otoh.
I dislike the attention people pay to what is “in style” for houses. Styles change and a home is the house you make into your own because you live there every day for years, not because visitors will critique it. I personally dislike most of what many consider stylish- to me baseboards are the bumper, not something you want to notice. I don’t think people should be focusing on the wall-floor junction but instead a few feet above that.
I do not care what people did generations ago. They dealt with different materials and techniques than we have available. Modern day Europeans are not stuck in the past- a pet peeve of mine is when on the home shows buyers/renters want something that is more “X” - homes and apartments are being built for the locals current likes, not what was done before. “Character” should not be equated with old and ornate either.
There- had my soapbox moment. Do the baseboards that appeal to YOU on a daily basis.
Yes, I thought you meant baseboard heating, not molding! Lol!
What’s in style can matter with resale. If one is keeping re-sale in mind, you don’t want stuff that makes your house look really dated. If it’s not important, then people absolutely should go with what appeals to them.
I’m not a fashion maven, but neither do I wear the same hairstyle I had when wings were in, I don’t wear bell bottoms or poodle skirts, or go go boots. Fashion does evolve, and we tend to evolve with it. Being a slave to fashion is another thing altogether.
How about no molding at all and have the wall run down to the floor? That’s what I am planning.
Goops…this is what I wear. I tracked down a few pairs of bell bottom and I think I am all set for a while
Works if your walls are adobe and a few other materials. NOT a good look with sheetrock.
Why not? I would think it is the interface between the wall and the floor one has to watch out for. It would require a skilled workman.
I would not skip baseboards with Sheetrock walls. Your housekeeper will have to be uber-careful about not bumping vacuum cleaner or a mop into the wall… If you ever worked with drywall, sheetrock crumples easily along the cuts where the gypsum fill is exposed. Also, watch out for any moisture… Baseboards are there for a good reason.
The baseboard moulding is the interface. 
There are techniques to interface without the baseboard.
What material are the floors?I have ceramic tile floors in my western house and whenever I have new tile floors put in (as opposed to the wall to wall carpet that was in the house), I have tile “baseboards” added --cut tile about 3" wide along base of the walls with a small amount of grout along the upper edge of the tile to hide any gap between wall and tile.
Some tiles have matching bullnose baseboard tile.
Super easy to clean and more durable/more scratch & dent resistant than wood.
There are some great ideas/images on Pinterest
“Your housekeeper”???
Oh, you mean me? 
@Consolation - it is apparently for an out of state rental. I assume OP is not going to fly there to dust the baseboards or mop the floors.
(No housekeepers here either other than you know… )
Yep, OOS vacation rental, so cleaned after each guest, but man, oh man, that red sand finds its way inside mysteriously. I don’t want to set up the house to look dirty on arrival if there was not a same day turnover & it’s unlikely to get someone to dust the baseboards any time there is a gap between guests, so no big flat bulky top ledge. Tile is a great idea, if I could/can get more of the tile that matches. I should have thought of that at installation!
As others have said baseboards are there for good reasons. Modern museums often don’t have baseboards. They’ll have a recess that is a good 1/2" deep and 1/4" high painted black instead. See for example: https://theculturetrip.com/north-america/usa/georgia/articles/a-history-of-atlantas-high-museum-of-art-in-1-minute/ Museums touch up their walls all the time. If you really hate baseboards you can cover the gap (and natural materials move) with a quarter round molding stained to match your floors.
Paint your walls and trim in red sand color.