San Francisco - but it s a harder walk because of all the hills. But you can see more in a smaller space in SF. Both places are great for walking though!
Even though San Francisco is hilly, this is an advantage for seeing things, because there are times when you are higher up and have a good vantage point. It s also a lot more compact than New York, plus there are less times when the weather might prevent you from walking outside comfortably. In New York, you are on flat terrain and amid giant buildings. So, you really only get to see what s at the bottom floors of the buildings as you walk down the street (unless you want to be all touristy and walk around staring into the sky - and still you re just looking up the sides of huge buildings).