I've traveled alot, and always chose to stay in small family run hotels or pensions. Cheaper (alot), and you get more of a feel of what the average person is really like in that country. Just my opinion, because I want to be with the natives when I travel.
I went to London last with no hotel reservations at all--like I've ALWAYS done in all the foreign travels I've had (23 countries so far). There are hotel accommodation desks in each railway or airport for travelers in Europe/Scandanavia who are only too glad to help you if you tell them your price range and what you are looking for as they get a commission on each booking they do. They will call the hotel or pension for you, and make the reservation for you on the spot. I have no idea what else they do as this is all I have ever used them for--other than simple questions like, "what time does the flea market open?"
They will tell you how to get to the place and will speak English, also, if that's a concern (they have to be fluent in English to work there). Different system totally over there than here. So, if you leave with no reservations I wouldn't worry about it one bit.
When you get there just tell the hotel accommodation desk that you want to be so close to the action as possible but in a SAFE area. You surely don't want to be near the tourist attractions that are surrounded by a crime area, and it happens. Again, they will speak fluent English, and my experience with them has always been positive every place I've been.
Both Paris and London have extensive innercity rail systems, so you might want to purchase cheap rail maps when you get there. And definitely use all your tricks to hide your money and avoid someone slashing your straps and running away with an expensive camera, tote bag, you wife's purse, etc. Lots of pickpockets and thieves target American travelers.
If you check some of the travel sites--assuming nobody here comes up with names for you to use that you like--there will be ratings and information on different places to stay in both cities. Lots of the net about this if you feel more secure with reservations ahead of time.