We will be arriving from Paris on the Eurostar around 11:30 on a Tuesday. Can anyone tell us the fastest way or least expensive way to Victoria Station? We will be staying at the Rubens. Would a cab ultimately be the least expensive - Tube, Bus? If so which route?
Take the Victoria Line tube south to Victoria. http://www.tfl.gov.uk/cdn/static/cms/documents/standard-tube-map.pdf
Using the tube, it will take about 8-10 minutes to get from St Pancas/Kings Cross to Victoria Station. I'm not sure how much luggage you'll have. This is the quickest and cheapest way to go. Checkout www.tfl.gov.uk for more information. It's about a 10 minute walk from Victoria station to your hotel.
We'd estimate that a taxi may cost 10 pounds. If that's easiest for you with your luggage, then go for it. The taxi stand is well-signed, right near the Eurostar arrivals area: http://www.stpancras.com/the-station/how-to-get-here/Taxis.
There is no direct bus route, although it is possible with one change. Not recommended.
I have no doubt that a taxi would be the best all around method for you. You'll have luggage which is a pain to drag onto the Tube and taxis are not that expensive in London. Stress free is a good way to begin.