I have a Super-Takumar 120mm f2.8 that I prefer to the 135mm f3.5 for portraiture.
I don't find the difference in focal length all that significant most of the time, certainly not for outdoor portraiture where I can zoom with my feet.
I would buy the K-mount version if I found a good deal, but it's low on the priority list.
The 135mm f2.5 Multicoated Takumar (1st edition) is better than either, but a lot bigger.
I also have a Vivitar 135mm f2.8 close focusing lens that is decent, but the colors are very different from what I get from Pentax branded lenses, so I don't use it much.
And I've had a lot of weird, off-brand 135mm lenses over the years that have been various levels of quality. I've heard folks say there aren't many bad ones... I disagree
That said, I like the Pentax M135 f3.5 best of the 135s because of its size, and the improved coatings over the Takumars help.
It usually travels with me when I take film cameras...
I used to have the Takumar (Bayonet) 135mm f2.5, which flared more than I liked.
But I would like it to be autofocus...
Most of the time now, I'll use the Rokinon/Samyang 85mm f1.4 for intentional portraits. It's a bit dreamy wide open, but it's very good otherwise. I usually use it between f2.8 and f4.
If I'm feeling contrarian, the K45-124 f4 is a very good lens for portraits if you want more than just a head... (it doesn't focus very close without help)
I'll also second the suggestion that macro lenses can be good for portraiture.
But really, I think it is at least as much about the comfort you have working with your model and the space available.
I've seen really good portraits with all kinds of focal lengths, so it's going to come down to how you want to shoot.
-Eric