You want - no, NEED, something in the region of 85 - 135.
Anything else can take portraits, but a 50 will not flatter the subject close in. The standard on 35mm full frame is a 90mm, so something between 85 - 135 is within those parameters.
The longer lens, with the SAME f stop, will render the background more out of focus with the same framing of the subject.
You need more room to use a 135 than an 85 (so if you are in a limited space, the shorter lens is the more versatile one.) 135 will allow you to remain further from your subject to get the same framing. The 85 will allow youork closer.
Hope that helps - OH, and buy the FASTEST lens you can. If going for an 85, see if there is an 85mm f1.4 (Nikon do one, so I expect Canon do)
Question: Do Canon do a specialist portrait lens? One they market specifically for that ? I don't know, because I use Nikon SLR and only Canon compact. Nikon do a specialist portrait lens - the 105 f2 DC, they also do a 135 f2 DC - and the 85 f1.4. Any of these would create fabulous portraits, given a decent operator.