Hi Naz,
Ultra-wide lenses tend to result in unflattering portraits - unless you intentionally want that crazy distorted look.
But for events they make a lot of sense.
Yes - the two lenses you mention are EF-S which only work on crop sensors.
The 17-40 and 16-35 are both good lenses, and if budget allows then the 16-35 IS would be my choice; no experience with the 3rd party lenses.
There is also the new 11-24 which is getting stellar reviews, but at a price.
For my wide angle shots I'm using an 8-15 fisheye. 8mm gives whacky images that really stand out, but most of the time I use it at 15mm. It doesn't have the stretched corner look of the recti-linear lenses mentioned so far, which is great; but the trade off is curved lines; I do occasionally de-fish the images in post processing. It's a good compromise and it suits my style.