Wow, so much negativity. So, first, I really appreciate the experiences of those who have used both.... you should to. There some folks here repeating stuff they've read. That's sad. Anyone can read and make their own decisions.
Quote: I played around with an old Canon rebel but I would like to buy a camera that would last me awhile and provide me with good IQ, good AF (not shooting sports necessarily but would hate to miss shots) and usable images in low light.
Don't listen to the guys above...a D7100 and K-3 have been tested in labs, they are practically identical in AF. The K-3 has better initial focus, the 7100 has slightly better tracking. The K-3 can focus in light about 2 stops less than what a D7100 can focus in. I mean the D7100 literally cannot lock focus with it's focus assist light, while the K-3 focusses without assistance. Ignore the above poster, they know not of what they speak.
Quote: I shoot primary candid photography (street photography, portraits on the fly) but would like the option to shoot wide for travelling (landscapes and monuments/temples etc...). Portability is also a major factor (for example I like shooting portraits but I'm unsure if I would buy a prime solely for that use). I would only shoot handheld.
The lens for that is the 21 ltd.
Quote: The Nikon prime is more expensive but provides a wider aperture which would be great for night shooting and the Pentax DA 35mm 2.4 is a bit slow in that aspect.
The reason the Pentax is slow, is because Pentax makes slow but very portable lenses.
The Nikon 1.8 is 200 grams. (next to nothing).
The Pentax is 124 gms, (2/3s of next to nothing)
I know a bunch of folks are going to say the Pentax is too slow, but it's 2/3 of stop, not even a full stop. I always say, if you can't fake a stop, you aren't a photographer. This falls well short of that kind of difference. Now ƒ4 to ƒ1.8, that's a difference.
What you can see there is the whole Pentax design philosophy. Stay away from really fast glass, find that optimal size where performance is guaranteed, but before weight begins to set it. Pentax does have the very expensive 31 ltd 1.8, which sounds like it's the lens you want, and it's well beyond anything you'll find in a Nikon and considered by some to be one of the best 3 lenses of all time.
Quote: For a walk around lens I was looking at either the Sigma 17-50 or the Tamron 18-50 for the Nikon and the Pentax 18-135 or Tamron 17-50 (if I can find it used) for the Pentax.
Another of the Pentax design philosophy for older Pentax glass, was extremely sharp centres, softer edges. This is a design born of the common " I want to take pictures of the people I live with" approach to photography. "Photography the way people take pictures, not for the test charts" is the way Pentax put it. Reading through this thread, it's amazing to see that not even one of the posters above, even seems to know what the Pentax philosophy was. Even 8 years ago, this motto was repeated to me by a Pentax rep at a trade show. Everyone wants to compare images on test charts. No one wants to just compare the images they take....anyway, enough digression.
The 18-135 is a great landscape lens from 18-50mm, and a great portrait lens, from 50-135 as well as a workable pseudo macro lens. As far as I'm concerned, it's probably the best walk around lens, ever. I own the Tamron 17-50. It's not a walk around lens, it's too short.I wouldn't recommend any of the 18-200-300 type lenses either, too much loss in resolution. The Pentax 18-135 provides the optimal compromise with a 7.5:1 zoom ratio. GO over that you lose IQ, go under, you lose range. And these days it can be had for a song. That being said, don't take advice on this lens from people who don't use this lens. If fit suits your style, you'll love it. My wife on the other hand doesn't like it. She'd rather shoot with her Tammy 17-50, and tamron 90 for length. If she needs longer she use the 1.4 or 1.7 TC on the Tammy. She clearly doesn't mind fiddling with lenses. I prefer a more "available" approach. Don't let the nay sayers hold you back when considering this lens.
Quote: The Pentax lens has a longer reach but is apparently soft at the edges and the IQ is only deemed "satisfactory".
That's over simplification. The 18-135 is rated excellent in the centre everywhere in it's range. Edges are weak in the longer FLs. It's probably the best Pentax lens ever made at 24mm. You won't find another zoom, 7.5:1 or higher that is excellent in such a high percentage of its range. So, it's either the best lens of it's type you ever used, if your subject is in the centre, or the worst if you value edge to edge performance above 50mm.
Pentax SMC-DA 18-135mm f/3.5-5.6 ED AL [IF] WR - Review / Lens Test - Analysis My 18-135 images here. 18-135 is a pseudo macro (notice how little the soft edges are not noticeable and imagine a person who would give up those sharp centres, to get a not as sharp centre but better edge to edge. I know those people exist, I just don't want to be one. And yet 90% of the people on this site recommend that, but it's not the Pentax way. Mind you the 18-135 was probably the last lens designed with the old philosophy. There isn't going to be another lens as good as it is for what it does. Either buy this lens or go another direction. Modern Pentax is starting to look very "me too." They used to present an alternative design approach, these days, not so much. But they are still making the best glass designed by the old design team, so you can still benefit from their work for years to come.
Quote: I'm concerned about the banding issue with the D7100 and the AF issue with the K3.
What AF issue? If you're talking about a D4s, a D750 or any other camera that costs 3 times as much or more, then there's an AF issue. If you're talking D7100 or D7200, you've been mis-informed.
Also ignore the frame rate nonsense posted above. Turn off tracking, which is in most cases un-needed, and you get the advertised frame rate. I use it all the time and it's awesome.
I assume because you asked here, you wanted someone to present the 'Pentax mindset." I apologize for all the Nikon wannabes who jumped in.