Neither. I would purchase a 70-200 f4 IS or if you can afford it the 70-200 Mark II. I had the 70-300 and purchased the 70-200 f4 IS. Sold the 70-300 to a friend. He borrowed my 70-200 one day. One month later he owned a 70-200 f4 IS. I don't know much about the 28-300 and the 70-300 is a very good lens. It is just not an L lens.
The summary is not surprising. A decent lens but the "immense zoom range does not come without compromise". Even without a review that would steer me away from it. But that is just me. I'm sure there are very happy owners out there.
I have the 35-350L (the earlier incarnation) and it is awesome. I prefer the added wide angle range and the IS of the 28-300L. I've used one through CPS loaner, and found it perfect for all-around shooting on full frame. It is heavy, but not as heavy as carrying all the lenses that would equal it. Same for the 35-350L. Both lenses have excellent imaging at f/8, and are even very good wide open.
If you are NOT interested in wide aperture zoom (f2.8) then the 70-300L is proved to be a great lens.
I do not own either so take my opinion with a bit of salt. The 70-300L is smaller, lighter and has better IS.
I would take that over the extra 28-70mm
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...
Jerry
28-300
The 70-300 L is an oddity: it's not fast, not ultra range, not compact/lightweight and not cheap. I feel one or more of these attributes may be better served by another lens. At the same time the advantage of the 28-300 is seen clearly.
