Hey Cleng,
Of those two lenses you list, I'd suggest the Sigma 70-200 f/2.8 will be the most suitable for indoor shots. The Canon 70-200 f/4L is a lovely lens (I own one) and produces some gorgeous results, but indoors that extra stop of light the Sigma will give you will be very important - it can double your shutter speed at any given time, and give you a shallower DOF which is often an advantage in these kind of situations. I haven't used that particular Sigma lens so can't comment specificall, but I've heard good things about it. My point is that the Canon f/4 will be fantastic outdoors, but be of limited use indoors.
Incidently, I have a few wedding photos taken with my Canon 70-200 f/4L in
My Gallery. The second-last shot on the top row and the first four shots of the second row are all taken with that lens.
But... I think it might be worth at least
considering a prime lens as an alternative, especially if indoor shooting is important to you. Even f/2.8 can be quite a limiting factor indoors, and quite often the flexibility in aperture and shutter speed can be more important than the flexibility in focal length. After all, its often easy to "zoom with your feet" to get the shot you want, but often not practical to change the lighting conditions in a room. And I try to avoid 1600 iso on the 350D whenever I can - there's a big difference in image quality between 800 and 1600 iso.
Of the primes, Peto mentioned the 135 f/2.0L that I have (except he included a typo - he mentioned it as f/4.0). This will be a whole stop faster than the Sigma f/2.8, so you'll be able to double your shutter speed (quadruple the shutter speed of the Canon f/4 in any given light) and it produces beautiful portraits in other regards as well. In that same gallery linked to above, the first and last shots of the 3rd row are both taken with that lens.
I'm also attending a wedding myself this Saturday and will no doubt be using both these lenses, so I might be able to give you a bit of a comparison between them both indoors and outdoors.
But I'm not sure how useful a long telephoto will be indoors... I guess it depends on the size of the room(s) and how close you can/will be, but Canon make quite a few fast primes in the 50-135mm range that are very sharp, and some are quite affordable.
The ultimate indoor candid lens I think would be the EF 85 f/1.2L, but its also over double the price of the EF 70-200 f/4L so probably not practical... While we're in dreamland the EF 200 f1.8L is on my wishlist too!
But for around the same price as a 70-200 f/4L you could get an EF 50 f/1.4
as well as an EF 100 f/2 (non-macro). If you splashed out a little bit more on a Kenko Pro 3000 1.4 teleconverter (which will work with both these lenses, unlike the Canon teleconverter and is less than 1/2 the price) then you'll end up with a 50 f/1.4, 70 f/2.0, 100 f/2.0, and 140 f/2.8... and all of a sudden things aren't looking so bad.... You're covering a similar range (just a bit wider) and for the most part you're at least a whole stop faster than the Sigma, two stops faster when shooting at 50mm... and three stops faster than the Canon f/4! You might curse at the inconvenience of changing lenses, but you'll curse more if you end up with blurry shots from using slow glass!
I know you already have an EF 50 f/1.8 II, so I guess you could use that instead of buying a 50mm f/1.4. But if yours is anything like mine then I wouldn't trust its autofocus for something as important as professional wedding shots. I have used my 50 f/1.8 for low-light indoor wedding shots, and while the good shots were wonderful, I lost enough shots through mis-focus to want something better if I were doing this stuff for a living. Not only does the EF 50 f/1.4 have much better autofocus and is build quality (and slightly better optics), its also faster than the f/1.8. And while its not the same bargain as the f/1.8, it isn't a mega-expensive lens when you compare it to other lenses.
Another alternative might be the EF 85mm f/1.8 which is apparently a wonderful little lens that always receives rave reviews it seems and great value for money. And with some non-Canon 1.4 teleconverters (it won't work with a Canon) it becomes a 120mm f/2.5 which is also very useful. But this option might leave you a little on the short-side when shooting in a big room or outside - even with a teleconverter. A 135mm f/2.0L would be a better option in this situation, but its more expensive than either the Canon and Sigma zooms you list.
Sorry.. I've probably confused you more! I haven't shot enough indoor stuff at weddings to know how critical it is to go faster than f/2.8... and obviously it depends a lot on the situation. Certainly both the zooms you list are lovely lenses and will give beautiful results.. and there whole zooms vs primes debate comes down to personal preference really.
But that's my 2c worth... or about 40c worth by the length of this post
good thing i'm just a backup that's why i have the luxury to shoot some candids. in your opinion, do you think the 4L can't hang indoor shots with just the available light? coz honestly i'd rather get the 4L for canon body compatibility in the future. but the problem is, i'm getting a tele lens for the purpose of getting candid shots and if the 4L can't serve me that purpose, i'm good with the sigma 2.8 for now and just worry the lens upgrade maybe next year