I have gone through quite a few 50s, either owned or tested or both, and the one lens that came out tops for me is the 50L, with Sigma 50 second best, IQ-wise.
When shooting close, the 50L is probably less sharp towards the corners, but considering the types of subjects one shoots nearby, that generally is not a problem, IMO. Further away it is tack sharp everywhere. And amongst others I use it for landscapes. It is a true standard lens on FF: it is usable for any kind of photograpy you can point a stick at, including macro.
The main difference between the 50L and the SIgma, apart from the aperture, is the "AF experience". I've tested only 2 Sigmas, vs 5 50Ls, but generally the focus of the 50L is very accurate, and the Sigma's just isn't. Both have focus shift in the nearby area, based on the two Sigmas I tested the SIgma has it a little less when it is most pronounced (3 50Ls with visible focus shift had it worse), but the Sigma has general focusing problems. It appears about 1 out of 3 is fine, from comments here and on other forums.
With the 50L the situation is probably not so much different, except that focus shift is the main problem, unless it is contained well within the AF limit of the body with a fast lens (1/3 of DoF), like it is with my specimen (actually, 1/6 of DoF). Occasionally there may be other problems with the 50L, but those don't seem to occur very often. Focus shift is something you can work around, to a degree, but I do generally recommend people to get a specimen from a retailer with a good return policy, in order to be able to exchange it for a good one, just in case. But then, I guess the same can be said about the Sigma.
And no, other lenses are not better, between F/1.2-F/2.8, unless you want to go F/1.7 or slower, and/or MF and/or alt lenses.
In short, in order to get a 50 mm lens with AF which is good between F/1.2 or F/1.4 and F/2.8, you may need a lot of patience.
With 35 mm it probably is a lot simpler. However, I for one do not gel with 35 mm, and i have heard the same from other people. I find 35 mm too short for a standard lens on FF, and too long to be a WA, and I just don't "see" the 35 mm FL. Similar things are true for me on APS-C, too long to be a standard lens, and too short to be a short tele. Neither fish, nor flesh. This is a rather personal thing, however. So if you want the easy way out, i.e., get a 35 mm, and since you say you like the 50 mm FL, I'd suggest you really try this out for a slightly longer period, like a week on end. Set your 24-70 to 35 mm for one week, and to 50 mm for the next, and base your final decision on how you get on with either.
If it is 50 mm, just get one, and return it if it has excessive focus shift, excessive for you that is, and get another one. It took me 5 specimens, two recalibrations, and about 18 months or so in total, including recalibrations, to get the one that I own in a condition that I am (very) happy with. And it is my most used lens now.
Kind regards, Wim
