Upload & Sell: On
I am currently renting a 70-300L for evaluation purposes; I find it to be consistently sharper at 300mm than my 70-300 DO, (which I've used for several years), and the IS is "steadier" and quicker to settle down; still, its white (or at least the newer grayish color), and also f/5.6 at 300mm. I suppose the slow aperture is the deciding factor in my purchase decision; today, for example, is a very cloudy, rainy day, and I won't carry the 70-300L with me when I make my afternoon drive into town. I just won't be able to get high enough shutter speeds without raising ISO past my comfort level. I may as well just carry my 100-400, though the newer 70-300L might be a tad sharper at pixel level, and of course the IS is a newer generation...
If I didn't already own the 70-300 DO, I would buy the 70-300L; however, its not worth the trouble of selling the older DO lens (they don't fetch much on the used market) and spending the extra $600 or $800.
I rented a 28-300L a long time ago; the "one-lens-covers-all" solution had its attraction, but I decided I'd be happier with larger apertures and multiple lenses. The 28-300L is a fine lens - I'd say Brian Carnathan (http://www.the-digital-picture.com/ ) pretty much nailed it in his review of the lens.
I'm obviously a huge advocate of renting a lens before buying one; at first it may seem like money thrown away, but just a few productive days with a lens in hand can show me way more than all the forum advice (no matter how good the advice is, and, BTW, welcome to the most honest, helpful, and respectful photography forum I've found on the net ) ever can. When I'm looking at spending thousands of dollars on a piece of glass and metal - to further my "hobby", no less - spending a few hundred to actually use one first has saved me LOTS of money and hassle...I'm sure there are several reputable rental companies, but I have experience with only one; I'm probably going to get in trouble someday with Fred for recommending them so often, so shoot me a PM and I'll sing the praises of LensRentals.com for you...
Let us know what you end up buying, and, once again, welcome...