From what I understand, the 35A and 50A are very similar in every way. Based on that, here are some observations from my first month of owning the 50A vs my L glass.
--The Sigma indeed is super sharp, as all reviews say. I know my PP work isn't the best around, but I can't reproduce that kind of sharpness in Post.
--Working in PS in CS5, I have to work to get L colors out of the 50A and I can usually reproduce them pretty well. That said, I find this to be the biggest difference.
--The 50A, once micro-adjusted, offers very fast and accurate AF. Not as good as my 135 and 200Ls, but close enough where I would be OK using it on the baseline at a basketball game.
--Construction of the Art line and L series is pretty good, although the Art line doesn't offer weatherproofing. But the 35L doesn't either, right? Can't recall.
--If you have a body that offers micro-adjust, do you really need the dock? I'm not getting it.
--After the $20 rebate from Greentoe, my 50A was $768 brand new and shipped to my door. I've seen the 35A in the mid $600 range refurbished, bottoming out at like $619 from Sigma direct. How much is a refurbed 35L, like $1050ish?
I labored over Sigma A vs Canon L for months, but my decision was made for me when my low offer to Greentoe was accepted. I'm very happy with the 50A and I would do it again, although I openly prefer L colors SOOC. Also, the 50A is a nice change-of-pace piece of glass for me as it gives a different look from the Ls. Lastly, the Sigma A glass thrives doing B&W conversions because of the contrast.
There are no bad decisions between the two, just different ones IMO.
Sony A99ii, RX-100ii, Sonnar T* 135mm f1.8 ZA, Planar T* 50mm F1.4 ZA, 24mm f/2 SSM Distagon T*, Minolta HS 200 2.8 APO, Minolta 2xTC APO, HVL-F43M