It's a pity that some seem to only consider portraits as those consisting of 'head shots'. Portraits also include three quarter/full body shots, family portraits, as well as group portraits, large and small. I've never had to do portraits where fast AF is an imperative; it's not like the subject is about to bolt unannounced.
For traditional portraits where you have a model standing and posing sure, but what about candids? Or less malleable subjects like small children? IMO fast AF can often be a factor in capturing a unique unplanned moment or expression, even with posed models, a quick laugh or a certain look, if you're not pre-focused you may miss it with a slow to focus lens.
I don't think anyone has suggested that portraits can only be of the headshot type in this thread. Personally for anything with more than say half body in the shot, I like a bit shorter focal length. For full body shots or group shots, I like 25mm or 17mm, sometimes even 12mm if its a large group. So a good prime(25/1.4, 17/1.8, 12/2) or a 12-XX 2.8 zoom is what I will grab in those situations.
Now, to the OP: If I wanted a prime for portraits, I would go 45mm, Nocticron or 75mm (I own all three and they are excellent). The 45 is a fantastic choice for the money, and the 75 is very good but you need to have quite a lot of working distance, to get a full body in the frame at 75, you need to be outside or in a very large indoor space. Of all 3 I like the Nocticron the best, because I get the working distance that I prefer with an 85mm-equiv and the shallow DOF like the 75.
If I wanted a zoom for the extra versatility, I would go with the Pana 35-100/2.8 because of the small size, weight, and fast AF. I don't see anything extra the 50-200 would give me for portraits over the 35-100 (portraits at 100+ (200mm-equiv) mean you need to have a huge amount of working space, and you're so far from your subject that its difficult to communicate, not sure its really useful for anything other than press/paparazzi style shooting), though it may make a good wildlife/general purpose telephoto lens going all the way out to 200mm.
The 40-150/2.8 seems interesting, but it looks like it will be double the size and weight of the 35-100, so I would probably pass on that. I sold off my FF gear +huge 70-200/2.8 and I'm not eager to get back to gargantuan lenses. For that matter, the 50-200 is roughly 3 times the weight of the 35-100, surely not an insignificant difference. The 35-100 and 100-300 would give you more range with less weight than the 50-200 alone, if you want a long reach lens for wildlife etc, just something else to consider.
