I took the 17-40L and the Canon 35F2IS to London last June. The 35 was useful for very low light and museum interiors. The wide zoom worked great for the sights. I would have been happier with the 16-35F4 IS, but I still won't sell my 17-40 to upgrade.
I prefer smaller and more compact lenses on full frame for a number of reasons. Here is one: when I asked a Yoeman Warder at the Tower of London if I could take his picture, he declined. He said that my 17-40L was "too much lens" for him. A smaller lens is less conspicuous, but it is difficult to get around this with full frame zooms.
IS is very helpful when lighting is extremely low, and while I missed IS on my zoom, and I missed the range of the 24-105L, a 24 was not wide enough for some of the places I shot. The 16-35 F4IS is well-worth it for travel, if you are overseas. I didn't really need a lens that wide in Paris, but you sure need it in Italy. It all depends on where you are going and what you want to photograph.