[dadipentak]

I say if you want a macro lens that can do portraits, go with the longer lens: if you want a portrait lens that can do macro, go with the 70mm.

I have the Sigma 70/2.8 and love it. I'm not a really serious macro shooter, though (when I want to pretend I am, I use the Sigma 105/2.8 which is also very good)

By way of a sample with the 70mm:

[Bob from Aus]

One thing to also consider is that the Pentax lens lacks an aperture ring. This limits its usefulness with film bodies and/or bellows and extension tubes.

That's when I get out my trusty 35 year old bellows and equally old Takumar 1:4/100 macro lens, which I have recently seen reasonably priced

[baro-nite]

With macro a longer focal length is preferable

Often, but not always. Where having greater working distance is important, such as when shooting insects, yes. But there are times when a shorter working distance is better, as when using flash when you want highly diffused light. With a longer working distance it's harder to get the light source as close to the subject.

When I chose the Sigma 70 a few years ago, like the OP I had in mind that it would double as a portrait lens. But in fact I find I prefer a fast 50 or 55 for portraits. For one thing, 70mm @ f/2.8 doesn't give a particularly thin depth of field (at portrait distance) for those times when you want that effect. For another, as Bob pointed out, brutal sharpness isn't always wanted in a portrait lens. I do a lot of macro so I use the 70 a lot, and I really like it. Niggles: no aperture ring, focus ring turns in the opposite direction from Pentax lenses, no quick shift, and I don't much care for the metal screw-in hood. It's still one of my favorite lenses.

For sample images, nearly all most of these (check the focal length in the EXIF) are with the Sigma:

baro-nite's Album: Macro & Close-up - PentaxForums.com