The Sheraton San Diego Marina is a bit isolated (it's on the far side of the airport from everything, and you'll wind up taking cabs or, as an alternative, use the hotel's shuttle to the airport and take public transit from there). A cab to Balboa park is about $10-12, and the price difference in the rooms will easily make up for that. Read
this thread, particularly post #21, for a description of the place. And the hotel has a free airport shuttle, which, again, could offset some local ground transit expenses.