My husband and I moved from New York City to London 6 years ago and have lived in Marylebone for nearly the entire 6 years. I don't consider Marylebone touristy at all, although it definitely is a weekend destination for shopping and the fantastic Sunday Farmer's Market. There is a great High Street with a nice grocery store, fish monger, cheese shop, butcher and every other kind of shop you can think of from Divertimenti which is a great kitchen shop, to Daunt which is lovely book shop, Rocco the chocolate shop, Providores New Zealand fusion restaurant, Cologne and Cotton for bed linens, the list goes on and on. With 1500 GBP/week to spend you can certainly find something fantastic in Marylebone which is also near both Regent's Park and Hyde Park depending on what end you're on.
Mayfair isn't too far from Marylebone, a 10-15 minute walk max depending on where in Marylebone you are coming from and as mentioned it is certainly lovely, quieter and more private. However, that quietness and privateness comes at a price. You would pretty much have to walk to Marylebone to find groceries unless you do all your grocery shopping on-line. There are a few upscale shops and some restaurants but you would struggle to find your everyday needs around the corner. You should be concerned about Oxford Street. It is packed with people 364 days a year. I am not a big fan.
If, as you said you don't leave your San Francisco neighborhood then Marylebone is by far the better choice for you.