A warm welcome to the CameraLabs forums. You've set yourself an interesting upgrade scenario!
On the face of it the EF-S 17-85mm f4~5.6 IS USM (Gordon's review here
) is the natural choice but it has some optical issues. I have had my EF 24-105mm f4L IS USM for a year now (review
) and have never regretted the purchase but I rarely need anything shorter than 24mm. If you are willing to change lenses to go shorter than 24mm have you considered the Sigma AF 10-20mm f/4-5.6 HSM EX DC (Photozone review here
, SLRGear review here
)? It's arguably a better fit with the 24-105 in terms of zoom range than the 17-40L and you get that ultra-wide angle capability. It's quite a bit cheaper than the 17-40L as well so hopefully that keeps the 24-105 in budget.
I have the IS version of the EF 70-200mm f/4L USM zoom but both versions are optically excellent. The EF 70-300mm f/4-5.6 USM IS has received a great review (here
) so I think you would be happy with either but, for my money, the IS capability would weigh heavily if I were choosing between the two lenses you mention.
By the way, I've moved this thread to the Canon forum and deleted your later duplicate thread in that forum. It's not a problem but your question is Canon specific so it is best in that forum. Hope you don't mind.
P.S. I have linked to several reviews above. Both the Photozone
sites also have reviews of a number of other lenses that you might be interested in.