First, I would suggest reading the review on this site for both lenses if you haven't already.
When comparing the Tamron 17-50 f/2.8 to the 17-40 f/4, there are several important points:
Advantages of the 17-40 f/4:
- Build quality.
- Brand name.
- Quiet focus.
- Large, smooth focus ring.
- Full time manual focus.
- Weather sealed (filter required).
- Does not extend while zooming.
- Compatible with full frame.
Advantages of the Tamron 17-50 f/2.8:
- Double the aperture, which has several important factors:
- f/2.8 allows twice the shutter speed (usable 1/60 vs unusable 1/30)
- f/2.8 allows half the ISO (usable ISO 3200 vs unusable ISO 6400).
- f/2.8 allows more control over depth of field and background blur.
- f/2.8 allows activation of more autofocus sensors (on 450D?).
- More zoom range on the long end.
- More resolution and contrast at all focal lengths.
- 55% less money
If you're wondering how the Tamron can be so much better at aperture, resolution, and cost, the reason is this: it's a normal zoom. A normal zoom is very different from an ultra wide zoom, which is what the 17-40 is.
I recommend the Tamron.