forseti-6

Hope I didn't jump in on this thread too soon.

You said you shoot landscape and event photography. In that case, I don't believe just one or the other will satisfy your need. As previously mentioned, you will need a lot of light to shoot events as much will probably be done indoors. As for the landscape shooting, you'll many times be stopping down to f/8 or f/11, so when it comes to that, you won't need the extra stop.

My suggestion to you, is buy the 17-40 for your landscape and buy a fast prime for your events (i.e. a 50 f/1.4 or even a 50 f/1.8).

I shoot indoor photography a lot and sometimes my Leica 50 f/1 isn't enough, so you WILL crave as much light as possible for indoors, and a zoom lens just won't be able to provide that.

Oh yes, I meant to say this as well, but some how it escaped my mind...
Since you will be shooting with a 40D, to really take advantage of the wide angle to do your landscape, strongly consider the 10-22 EFS. While I have not shot the 10-22 myself, I have certainly read great reviews about it, and some going as far as saying it's Canon's best wide angle lens. Keep in mind, shooting a 17-40 on a 40D is essentially yielding only 27-64 mm due to the 1.6x crop factor of the smaller sensor, where as the 10-22 will be 16-35.

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