Definitely 5 days in Paris....or all 5 in London. Don't split the time between the two. I'm so not a fan of drive-by, checklist tourism. Better to stay a bit longer in a great city and soak it in, rather than hurrying from one sight to the next.

In Paris, you can often get an apartment at prices comparable to a hotel...which means you get a kitchen and washer/dryer (so you can skip the checked bags, always a plus). Beware that Paris hotels can make NYC ones seem spacious and palatial...

Paris has wonderful food shops and outdoor markets...if you like to cook & eat, it can be very nice to play at being Parisien and shop a little to make simple meals a few times during vacation.

I've used Paris Nice Home to rent before...the manager meets you either to hand over keys or to collect payment when you arrive. The apartments in the ville Montorgueil or in the Marais are very well sited for tourists. LINK /

Money is easy: order a few euros from your local bank so you have a bit of money in your pocket. While there, use your debit card to get more euros. Your chip credit card will work, but you'll have to sign, so it might take a minute or two longer for an establishment to handle your card...but most tourist friendly places are quite accustomed to our non chip n pin cards and will not bat an eye at the signature required stuff.

Paris is well situated for a few easy day trips...a short 45 min train ride to Versailles to tour the chateau & gardens, a slightly longer trip out to Monet's house & gardens in Giverny....or to Rouen, Riems, or Fountainbleu. So don't worry about "seeing everything" in 5 days (which is completely a joke in Paris), as you can take a short train trip if you want.