I faced the same delima when I got my 50D, not sure what type of photography I wanted to move into. I was looking for something that would not limit my shooting, while I was learning, while still creating great high quality images.
After agonising over countless flicker groups and comparison reviews, I decided on the 17-40L for the following reasons.
- On the crop body, its long enough for a good walk around lens, sure a little short, however, not short enough that you just need to take a step or two closer to your subject.
- The L glass is super sharp and excellent quality.
- The 17 -40 wont break the bank compared to some of the other L glass.
- If (when) I decided (can afford) to go FF I now have an fantastic wide angle lens rather than a EF-S lens I have to sell.
I've been shooting for the past 10 months and have never been happier, able to do 95% of what I want without a regret. The only lens I've added to my kit is the EF 35mm 2.0, I was originally looking at the 1.4/1.8 50mm, however on the crop, the 35mm becomes pretty much 50mm which I find is such an excellent all round prime/portrait/walk around lens by itself.
The way I see it, buy quality glass and you'll never be disappointed and it will last for the long term rather than until the next latest/greatest senor comes along.
Good luck and happy shooting and congrats on the 40D, I love my 50D, only the 5D would replace it.
Korbin,