If you were me, which lens would you buy? (24-105 vs. 70-200 f/4 IS)

[neuroanatomist]

At one time, the EF-S 17-55 f/2.8 IS USM was in this race, but I think I was scared off by the price vs. build quality (constantly read about dust issues), the (perhaps overly hopeful) thought that I will be upgrading to ff in the not to distant future, and the focal length range being slightly wide for most of my use.

The build quality is very good - it's a very sturdy lens (quality seems similar to my EF 100mm f/2.8<span style="color: red;"]L Macro IS USM lens, which though an L lens also has a plastic barrel). I think the dust issue is overblown - I haven't seen any problem, personally, and I think the relatively small number of people who do are very vocal about it. But as you say, any EF-S lens useless if all you have is a FF body. It certainly needs to be 'paired' with a longer lens, like a 70-200mm, 100-400mm, etc. I should note that while it's the lens that's on my camera most of the time, most of my shooting is of my family - around the house, etc., and the zoom range does very well there (I have an EF 85mm f/1.8 for indoor close-ups). I think what you're really paying for with the 17-55mm is the optical quality, which is excellent.

One of the major reasons I like the fast aperture is the subject/background separation (because most of my shots have been outdoors with plenty of light anyway), and from what I've seen in the sample pics of the 24-105 and 70-200 even more so, it seems like you can still achieve that with f/4 on a longer lens.

When you look at sample pics, be sure to pay attention to the camera used - the 1.6x FOVCF applies not only to angle of view, but also to depth of field. So if you really like the OOF blur in a sample pic at f/4 on a FF body (all of Bryan's sample pics with the 24-105mm are on FF cameras), keep in mind that will be like f/6.4 on your XSi in terms of depth of field.

It's kind of funny how my choice on this lens effects what I get for my next lens too.

So very true!