I'm thinking of getting either the Canon EF-S 17-55mm f/2.8 IS or EF-S 24-70mm f/2.8L lens. What are the main similarities and differences between these two lenses, and what are their relative advantages and disadvantages when used for social and wedding photography?
My guess is you're talking about the famous Canon EF 24-70mm f/2.8, as others have mentioned, there is no EF-S 24-70mm. This lens is bread and butter for a lot of wedding photographers and they will vouch for its quality. As a matter of fact, this lens is so popular that canon has released an improved EF 24-70mm f/2.8 II. Whether this lens is a good buy or not for you, depends on a lot of factors such as, your camera body, your budget etc.
The APS-C alternative to this lens is Canon EF-S 17-55mm f/2.8. Theres also a Tamron 17-50mm f/2.8 which is a cheaper alternative to this. You always get what you pay for.
Having owned the EF-S 17-55 f/2.8 IS USM, I can vouch for how good it is. Really an L series lens but in a plastic body and without the L moniker. The IQ is stunning. I have heard some 'horror stories' of the image stabiliser on it being flaky and failing, though personally I never experienced any problems in all the time I had it.
The main issue with that though is that it will of course - being EF-S - only work on APS-C sensor bodies like 1100D, 500D, 550D, 600D, 650D, 60D, 7D, etc. It will not work on any Full Frame camera bodies such as the 5D series, 1D series, or the newly announced 6D.
So if you are shooting with a crop-sensor body then I do recommend the EF-S 17-55. If you search this site you will also find references to how an EF-S lens on a crop body can actually be sharper than an EF lens on a crop body.
Re: the EF 24-70 f/2.8 L USM - I don't own this, but do currently have my name down with a 2nd hand shop for one if they come by one. The reviews of this lens are outstanding, and for Full Frame photographers, it is a fantastic general purpose lens as well as being ideally suited for wedding photographers.
Canon have just released an updated version II of this lens though, so you should see a few "bargain" mark I versions coming onto the second hand market at reasonable prices.
Either lens will serve you well, and you shouldn't notice the lack of IS on the 24-70. The main decision here will be as to whether you need to use it on Full Frame or not.
As Mark said, there is no EF-S 'L' lens. perhaps you mean EF 24-70mm f/2.8L II USM which is a none-IS lens, it's not much useful for low light conditions where you can't use a tripod.
It would be helpful if you add more details to your question, are you going to buy these lenses? or do you already own them? do you really need such a fast lens?
The 24-70 is about $2K, with this price I think you could have better choices, such as Canon 24-105mm f/4L IS EF USM AF.