Unless you need to save every dollar, take a taxi the whole way. BART from SFO is a rip-off. They have jacked up the rates, and overall it will take you MUCH longer to get to your hotel. Also, the short taxi ride within downtown will cost you more than you think. I'd strongly favor taking a cab the whole way.
BART certainly won't take
much longer, depending on whether there are events going on in the City, but I too recommend taking a taxi. Should be closer to $40 IIRC, given the Sutter St. destination.