Steam.
Urghhh, can't imagine why you'd want it.
I have the displeasure of owning 3 titles (I purchased the physical copies mind you) that run through steam and It gives me nothing but trouble. Now, when I'm considering buying a title, steam requirement is a major consideration against making the purchase.
The only selling point is auto patching but to me this is a big draw back too. If I see a flaw with a program I'll find a solution as I see fit, not because Steam tells me to.
Buy the disk, It's very cheap and you can decide when and how you handle your software.