Tough call. I prefer Paris, but my teenage daughter prefers London (it's hipper). We stayed at the Colonnade Townhouse in London which was very nice and reasonable by London standards. In Paris, we rented an apartment on Ile St. Louis, which was great and definitely cheaper than staying in a hotel. I say Paris and do a day trip to London for him to get a taste. It's easy by train.

Either place, consider renting a flat which you can definitely get for under $300.00 & save a lot of money over a hotel plus it's nice to have a small kitchen for snacks, breakfasts, etc. not to mention a washing machine.

This site has apartments in both places & I've used them a lot:

http://www.londonguestsuites.com/