Relative to Paris, I'd suggest reading the thread on the Hilton Opera before staying there. The reviews since opening have been less than stellar; by comparison, both the ren vendome and prince de Galles are quite well thought of. I'd opt for one of those in your situation and disregard the Hilton (I usually discount breakfast when in Paris as I Iike to go to a neighborhood bakery, but I'd likely choose the Ren anyway as I like the location). There's also an extensive thread comparing London Hilton properties; in London you have some nice choices including Bankside, Canary Wharf, and Tower, all of which compete for "best lounge" status (and the Conrad of course). Since you're interested in the area around the Bankside you might as well stay there and enjoy your Diamond benefits.