ATM cash or VISA for purchases

I would advocate for getting cash and using the card for large purchases or places where it is convenient. You need cash for smaller, everyday purchases for places that don't take cards - like mom and pop shops or market vendors. If you're really confined to big cities rather than small towns then most places will take credit cards but small restaurants, cafes or the wonderful street markets of Paris you'll want cash. If you're travelling between London and Paris you'll be switching currency so there is a certain argument to be made not to get too much cash or you could get stuck with it depending on your plans. Also be aure to tell your bank and credit card companies when and where you'll be out of the country. Hope that helps,
=Tod

Absolutely use the credit card. You will find a few places in Paris, and probably fewer in London, which do not take credit cards. That would be a few street vendors and bakeries in Paris. I would get about 100 euro and 100 pounds to use when required. But, whenever accepted, use the credit card.

Read the fine print in your card agreement. Many credit and ATM cards impose a substantial fee to do foreign transactions. Paying $5 for each purchase can add up in a hurry, especially for small items. Cash is convenient and, if you withdraw a large sum from an ATM once, it spreads a flat fee out so it is relatively less expensive.

I can't imagine using a Visa card all the time,, first off I would hate to carry it around all day , I leave mine in the hotel safe , ( or in transit its in my moneybelt) with excess cash. I never stress about pickpockets because I only carry a days cash around with me normally so any incident wouldn't ruin my trip.
Second I wouldn't use at street crepe stands, ice cream kiosks, for postcards, I mean I just use mine for big purchases, a spurgle meal out, and hotel and train ticket,, not for tiddly stuff , and there is lots of tiddly stuff .. lol Frankly I think that 2 dollars transaction fee is not such a big deal, I mean, you only need to take cash out every 3-5 days depending on your spending habits, I have my limit raised and take out as much as I can at once ( and then as I said I store it ) .The ATMS in both London and Paris do not charge any fee at all.

This article 'Pay with Plastic or Cash?' by Rick makes some good points: http://www.ricksteves.com/plan/tips/pay-with-credit-cards-or-cash.htm A guide I had recently said generally speaking Europeans don't use credit cards for purchases under €20.