"Night and interior" + "walk around" = 35mm 1.4L!
The main alternative answer to this equation, to me, is a fast 50mm, and as you already have a 50mm 1.8, you are already familiar with that focal length. My 50mm 1.8 is pre-Mark II, and so excellent, I have never seriously considered a 1.4, though I did purchase a 50mm 1.2L. This one is probably going to be a favorite on my newly-acquired pre-owned 5D. On a cropped-sensor camera, a 50mm can be a bit long indoors, depending upon how tight the dimensions of the rooms.
My 35L rarely leaves one of my 7D cameras, while the 100L is virtually welded to my second 7D. The only zoom I use much is my most excellent 10-22mm EF-S, which performs amazingly well in low light, but not as well as something that can open a bit wider.
I have no personal experience with the L zooms starting at 24mm, except to know that the 24-70mm 2.8L is large and quite heavy.