Trying to decide on a 50mm lens is tough! in my opinion there is no really good option.
I rented the Art lens a couple of weeks ago. Yes, it was sharp wide open. If that is someone's only criteria, I think it is the best option. All the other 50 mm lenses available in the EF mount have problems either with autofocus or, in the case of the very expensive Zeiss, no autofocus at all!
Note on the 50 Art, for lots of people, including me, you will probably need to use only the center focus point or those near it. The outer points front focus horribly. Maybe that will be a problem for you and maybe it won't
I've had the Sigma 50 Classic for a few years and got mildly frustrated with the somewhat erratic autofocus. If you use it wide open, you had better double the number of shots you normally take and make sure you refocus for every shot. That's what I do. At least it is fairly sharp wide open when you get the focus right. I did an outdoor photoshoot with the Art and the classic and had a hard time telling the photos apart when both were used wide open.
I just did a complete studio shoot with only the Sigma Classic on my 5D3. I used it between 2.8 and 5.6. I really enjoyed pixel peeping, it looked like an L lens to me.
I haven't tried the Canon 1.4, but it would probably be a good option for your 50D. I believe that body does not have MicroFocusAdjustment, which is a must for a non-Canon lens. It could even be an issue with the Canon 1.4 if you intend to use it wide open.
So whatever lens you get, be prepared for it to be less than perfect in one way or another.
A good photographer learns to work around the limitations of their equipment.