For a short five day trip, is it worth going to Paris, London, or both cities? Although they both have things to offer, I found that Paris easily wins the battle based on its enormous amounts of attractions, cheaper cost of living and admissions fares for said attractions, and overall fast-paced liveliness. I’d imagine in the summer, the lines may change that impression, but at least for the winter Paris seems to be the place to go, or to heavily weight your short trip.
Otherwise, airfare to Europe can be surprisingly cheap during Winter and Spring season (late December non-withstanding), even during Thanksgiving holiday. I ended up using STA Travel/StudentUniverse to buy discount one way tickets to London, at $235 o/w nonstop from Los Angeles on Virgin Atlantic. These are usually mileage accruing. For the return, a non-stop flight using 20,000k American Airlines miles + $80 taxes/fees from Paris on Air Tahiti Nui fit the bill. In general, it seems that one-way tickets to European destinations are particularly cheap on Student discount sites; then, for the return one can match off-season one-way fares back through OneWorld (American), or Star Alliance (United). Be sure to call to obtain best partner availability. Also, look into returning from Spain rather than France or England, as taxes can range from $30 in Spain to $150 from London).