[Purevulcan]

So say I only have the budget to get one of these lenses and Im looking for a fast "walk-around" (a "nifty fifty") lens for things like street photography and portraits. Which is recommended?

Pentax 35mm f/2.4
Amazon.com: Pentax 21987 DA 35mm f/2.4 AL Lens for Pentax Digital SLR cameras: Camera & Photo

I know that this is technically a 50mm lenses on APS-C but it has a slower aperture.

or

Pentax 50mm f/1.8
Amazon.com: Pentax DA 50mm f1.8 lens for Pentax DSLR Cameras: PENTAX: Camera & Photo

This lens is faster but also an equivalently around 76mm.

[alamo5000]

I can't tell you what is going to be right for you and your style, but I have found that a wider lens is better if you are just walking around. Of course there is such a thing as 'too wide'...

A 28mm on to probably the 35mm lens and anything you can find in between will be your prime time walking around lenses. It all depends on the effect you want though. You will find that the 50mm is kind of tight and the 28mm focal length a lot more forgiving as to being able to quickly frame up shots and so forth.

That being said, you can easily walk around and produce fantastic results with almost any focal length you put on the camera. It just depends on what you are after.

[SpecialK]

For portraits, the 50.

[hcc]

My second lens and first prime was a fast fifty (VL 58mm f1.4). It was one of my best decision and selection. To date my VL58mm f1.4 is my most used lens.

As point by specialK, the 50 is also good for portrait, although I tend to refer a slightly longer focal length.

I think that your initial selection tends to ignore the big difference between f2.8 and f1.8 or faster. The larger aperture (f1.8) will give you more option in low light in my opinion.

Hope that the comment may help.

[mtux]

For walkaround, the 35mm

[Na Horuk]

For portraits and low light photography, the 50mm.
For a more convenient angle, the 35mm.

Do you have the 18-55mm lens? Just zoom it to 35mm and then to 50mm and think about which angle suits your purposes more. Or if you need that extra brightness, go with the 50mm. They are both good lenses on a budget, I doubt either is "the wrong choice." I would choose the 35mm first, because it is a more convenient angle of view, unless I needed that f1.8.

[Kozlok]

For me, indoors a 50 is usually too long and I can't back up enough. Outdoors a 50 is usually fine. For me personally I did a series of tests with my zoom to see what FL I preferred. I found that I either preferred the 35mm length or my preference skipped to 70. Almost never did I "prefer" a 50mm shot. Go out and take the same picture with your zoom set at 35 and 50. Move around to frame each shot specifically for each FL. Do this 10 or 20 times, look at the results.

One more thing, if you get the 35, you can add a cheap 50mm manual focus (the Takumar 55/1.8 is a favorite of mine in this FL for ~$40, or an M50/1.7 is ~$50).

[Purevulcan]

For me, indoors a 50 is usually too long and I can't back up enough. Outdoors a 50 is usually fine. For me personally I did a series of tests with my zoom to see what FL I preferred. I found that I either preferred the 35mm length or my preference skipped to 70. Almost never did I "prefer" a 50mm shot. Go out and take the same picture with your zoom set at 35 and 50. Move around to frame each shot specifically for each FL. Do this 10 or 20 times, look at the results.

One more thing, if you get the 35, you can add a cheap 50mm manual focus (the Takumar 55/1.8 is a favorite of mine in this FL for ~$40, or an M50/1.7 is ~$50).

I like the 35mm FL and I like your idea about the cheap manual prime lens, what adapter do you recommend for M42 to K mount and where would I shop for the older lenses?

[Na Horuk]

For the Pentax M 50mm f1.7, a well-liked lens, you don't need an adapter. In fact, it costs less than many 50mm m42 lens + adapter.
Where can you buy it? Depends where you live. Maybe there are some used camera gear stores where you live, garage sales or pawn stores can also be a goldmine. Online, you can look on keh.com, the Pentax Forums Marketplace, ebay, or any local "trade/sale" websites.
Just remember that the M series has no AF and no auto-aperture. If you get a Pentax A 50mm, you will get a lot more functionality (no AF, though).