First thing I personally feel IS Technology is slightly overrated. It will not function as you may thing, a few stops is not as much as you may hope for. It is a lot less compensating but works very, very well.
I shoot in studio, along with portraits out doors with strobes, so I do not need the IS or the extra stops. I like the F/4 version because the contrast is so so good.
The price is unbeatable for the F/4 version, You can get this version if you like and you can always upgrade to the next if you feel it is inadequate; the resale value is great on this lenses. The focal length is tight on this lens so you will need some room to work it, but it works superb as a portrait lens.
Lastly these are all tools and they all work flawlessly. Budget and functionality will decide. I will say be true to yourself, and try to focus on what you need and not what you can get.