« on: May 17, 2013, 12:10:11 AM »
It depends on the sorts of focal lengths you like using, how much equipment you want to carry around and for how long, and what you'll be shooting. The days are long in both cities in the last week of May, so unless the weather's really heavily overcast you shouldn't have to worry much about low light unless you want to photograph inside churches etc., some of which are pretty dark inside even in the middle of the day. Moreover, with your 6D you may well find that a slower lens with IS yields better results in low light than a fast lens without it, so you could lighten your bag by leaving behind fast lenses (unless you want them for other reasons).
I can't quite tell from your post, but if you're planning on taking just one lens, would 24-70 II be versatile enough for you? If so, go for it. It would be too short for me. When I was in Paris a year ago, around the same time you're going, I took my 5DII and 17-40, 24-105, 70-200 f/4 IS (if I had had my 70-300L I would have taken that instead), and three 1.8 primes, but except when I was in Notre Dame or wandering around after dark I never used the primes and only took with me either 24-105 or 17-40 + 70-200. (And, as it turned out, the IS zooms generally performed better than the primes in low light (the results would have been even better had the 6D been available at the time). The next time I go, unless I just take my Olympus OM-D gear instead, I'll take 6D + 24-105 + 70-300L (both perform really well in low light on the 6D) and leave the rest behind.
If you're really set on the 24-70, you may want to supplement it with a 70-200 f/4 IS or 70-300L (I guess you could go whole hog and get the 70-200 2.8 IS II, but who wants to cart that around all day?).