Spending time in Paris and then to London

I would look at 7 nights in Paris then 5 nights in London - time enough for in depth exploration.
Allow for a couple of days trips from Paris, and one from London.

Unless you know that you have a lot more to see in one than in the other, an even amount of time in each is good. If you have an extra day, spend it in your first city, because the day of arrival is often a jet-lagged haze. Both cities have lots of day trip possibilities if you want a change from an urban experience or if you end up not liking the cities themselves (hard for some to imagine, but it does happen).

Do look at prices of airline tickets and hotels for both scenarios - starting in London and going to Paris, then vice-versa. Yes, the taxes departing London airports are higher, but other factors may make the total price better the other way.

You will want to get between the two cities by Eurostar train - much faster and easier than flying. You can book this ticket six months in advance. If you can book far in advance and accept a non-refundable ticket, it's very affordable; if you need flexibility or try to buy at the last minute, the price can skyrocket, and some runs can sell out. So, book as soon as you are sure of your plans, at http://www.eurostar.com/us-en

Paris is a much more compact, walkable city. Most of the attractions are contained in a 3 by 5 kilometre rectangle - you can walk from the Champs-Elysees to Bastille in about an hour.
London is much more spread out, and it's difficult to walk to the attractions, but the transportation system is excellent.
I'd spend more time in London, just because it will take a lot of time to see what most people want to see.
I'd also advise reversing the order, and fly into London and out of Paris. You'll save more money, as well as not suffer too much culture shock.

Hi,
I agree with many of the responses. We did a similar trip and split the time between the two cities. In London, we spent four days in the city alternated with day trips. In Paris, (with the exception of Versailles). I would stay in the city. I would take Eurostar to Paris instead of flying from London and going back out to Heathrow. It is not cheap, but it is an interesting experience and easier than flying.
Have a wonderful trip................JR in Orange

Each city is worth as much time as you can give it. I prefer French restaurants. Absolutely use a multi-destination search function for your planning. Don't bother about airport taxes. Unless you are flying on points, the taxes (and airline-imposed fees) are part of the bottom-line price for the ticket and that's all you need to compare itineraries. I like the tunnel train for the reasons given above. Buy your ticket as soon as it is offered because prices can double, or even more, the closer the purchase is to departure. Same thing applies to one-way flights on the budget airlines if you choose to go by air. Basic research tools:

www.seat61.com

www.skyscanner. com

I would tip it 5/7 towards Paris and fly into London and out of Paris as the fees for London airports make it a bit cheaper this way. 4 days is plenty for me in London and there is never enough time in Paris.