London.

It would be easy to spend a week at each with frequent trains and flights between them. My personal preference would be Paris if I could only choose one, but neither one is BETTER. It's just a matter of opinion.
Happy travels!

If you have two weeks you can easily visit both. It's only a 2:20 minute train ride between the two cities and the trains run very often.
I would spend a week in each and visit both of them. One week in each is enough time to see all of the sights and do some day trips (Versailles, Cambridge, etc)
Go to Eurostar.com to book tickets - they are cheaper the further in advance you book.

Paris

Well buddy, I believe that LOndon is one of the best destination to spend 2-weeks in Vacation. I am sure you will come back with lots of memories. This city is a home of plenty of attractions.