-For SFO to SF (city) I've seen estimate for taxi fare closer to $50, including tip, but have seen many other estimates, as well.
-SFO is, of course, the designation for the San Francisco Airport, though some people use it to refer to the City of San Francisco, a few miles to the north.
-There are lots of shuttle van services that go from the airprot to San Francisco. I've not used one. I've seen varying reports on reliability and service.
-One other option is the Caltrains (commuter trains) from near SFO. But then you'd take a taxi at the end to get to your hotel.

OP says 21:30 landing, not 23:30.

I am not speaking from experience, but I think the trip to Union Square, assuming no traffic, would be closer to $40 than $50, even with 15% tip. (Google Maps says my house is about 0.5 mi. further than Union Square, and I generally pay in the $40 range.)

Here is a link that provides information about taxis from SFO:

http://www.sftravel.com/taxi_san_francisco.html

The $50 estimate is about right. I have taken the cab several times from SFO to my office downtown, and it has ranged from $42-$50. At night with no traffic, you will likely be at the low end.

Taxi from SFO (the airport) it will be. In this case money is not an issue. Given the length of the flights and the time of night, I think it will just make life a lot easier to get in the cab at SFO and be dropped off right outside the hotel doors. Thanks again everybody.

darn that 24 hour cloick!
 

9:30 PM out by 10..I'd probably take BART then....altho i see you are doing a cab. that's fine too.

I regularly fly in from LHR and take BART. I find after a day's travelling, I need to stretch my legs and walk a little - Powell is slightly uphill to O'Farrell, which makes it even better from that point of view. Having said that, BART from the airport is painful in its irregularity and, since the increase in price, has become less of an attractive solution.

As money's no object, take a cab, check in and then take a walk around Union Square, up towards Nob Hill and down again just to ensure that you get a good night's sleep.

Do one, then the other
 

If it were me, I'd take the taxi to get to the hotel. Then, if you want to learn the route so that next time you take BART, take BART to the airport. That way, you know what the area is like and what to look for. The cab ride is only 20 minutes.

This has been my experience as well ($45-$50 range with tip). I usually stay at the Westin on 3rd (just east of Powell and South of Market). I've done BART from SFO and OAK when I've been traveling for pleasure and carrying minimal luggage, but when I'm traveling for business and expenses are covered, a taxi is a no-brainer.