I recently got both of these lenses as part of stash of lenses I bought off of Craigslist.org recently.  I don't see why I need both, so I decided to run some comparison tests using my new 7D.  Here are the first sets of results: http://newells4.blogspot.com/

Does anyone have any advice regarding these lenses?  Do you have and/or recommend one over the other based on your own experience?  The Sigma seems warmer and softer.  It also has Macro, which I haven't had much experience with.  I'd appreciate any thoughts.  I know they are both fairly cheap and are both older models, but I intend to keep one of them for the time being.

Yes, my opinion... is sell them both and get the 70-200, whatever the best version is that you can afford.

You can start at around $525 used (maybe better?) for the 70-200 Non-IS f/4 which is just as sharp as the 70-200 IS-USM f/2.8

It's smaller than the f/2.8, no tripod ring needed, shoots just fine and for what you give up from 200-300mm your shots will pop out clear and bright.

7D with a kit lens? Why! Would you buy used tires for your new Porsche? 

Want cheap macro? Extension tubes and use your best lenses.

If you really want macro the Canon 100mm Mark I Non-IS f/2.8 which has just been "improved" is a fine lens. With people jumping on the new version because of IS, you'll be likely to find one used for a reasonable price. Same thing, $400 range, but it's not the zoom you asked about, it's just a very nice 100mm f/2.8 True Macro lens that works great as a 100mm Prime.

Stick with the zoom, because once you see the shots from a prime, you will think all your old lenses suddenly went bad, overnight.  If you can find a store and do a test with a 70-200mm Canon, you may also find you want to sell the Sigma and kit lenses ASAP. See for yourself.

I appreciate your comments, they are very valid.  I can't wait until I can afford to upgrade.  I got these lenses (along with a couple others) for a steal, and think I can make a profit selling just a couple of them.  Hopefully in a few months I can make some upgrades.  I'd like a couple primes, maybe the Canon 35mm 2.0 or 50mm 1.4 to begin with.  And then maybe the 70-200 f/4L like you recommend.  Any others you'd recommend?  I appreciate the advice, being somewhat of a photography novice anyway, and having only very recently switched to Canon from Nikon, I have much to learn about Canon gear.

If you aren't looking for a zoom, you're already headed the right direction. If you don't mind changing lenses and can get all primes, you'll be even happier with results.

Nice that you got a good deal. I wasn't saying that the f/4 was the best lens to get, it just fits the same as what you have and low cost.

Do you need a 1.4 for some reason? If not the 50mm 1.8 is a good price and good value. Considering you have a crop camera, you may want to look for something wider than 35, at least 28 has the same field of view as a 44mm on a full frame. See where that's headed? You want something like a 17mm or 18mm (just numbers) to equal the field of view on a crop sensor, that would appear the same as a 28mm on a full frame camera.

I carry zooms for walking around, travel and changing scenes, portability, flexibility. In the studio you don't have all the variables and much less dirt and dust. I have a 24-105 and a 28-135. Hardly any difference, even though the first one is supposed to be a better quality product. 70-200 f/4 or the 2.8 both have visually perceptible better image quality. I had the 70-200 IS-USM and sold it because it was too big, then I bought a used Non-IS f/4 and 400mm f/5.6 and had money left over for an 580EX II. Just fits my needs better. Someone else might find the f/4 can't fulfill their requirements. You need to decide for yourself.

You can read opinions, reviews, tech sheets and everything else, but in the end, you need to match your lenses to your individual needs.