Matt, it sounds like you're learning a ton. Nice job.
I had the 24-70 sigma. It was alright, but I think the tamron is tighter. The lens that never leaves my camera is the 17-40 tho, I'm a wide angle landscape nut. I guess I have a thing for cheesy calendar photography. heh. The 17-40 is an alright lens, but it still gets a little soft here and there. I'd like to get the new 16-35L (I think thats the new one) that canon just put out to replace the 17-40, but life just changed a bit for me and it looks like photography is going back to being a hobby instead of a career.
Are you taking a color class? My first color class we shot slide film in, but this was before digital. Slide film was expensive back then, and even more expensive to get it developed, but printing color from slide film is much easier than printing color from color negatives. At least in my experience.
That being said, I don't think I'd ever go back to film. The resolution and all around picture quality of digital keeps getting better and better. I have my photos as a screensaver on our tv. The tv is huge and the pictures look great on it...now I just need to take a picture worth framing and displaying giant size.