I just want to add a couple of points. Filters on wide and ultra wide angle lenses - in particular circular polarizing filters - potentially DON'T. The reason why - beyond about 28mm, the angle of view becomes so wide, the polarizing filter can not affect the entire view (its just a matter of optical physics). The effect is most noticeable on the sky. The result is an area that it does affect, with the remainder of the view getting progressively less of the affect. Now, if you like the progressive effect and not a uniform application - then by all means go ahead.
Now, I understood the problem and went ahead with a CPL for my 12-24. Its a 77mm filter and they are not cheap - especially the thin variety. However, if there is a lot of sky - I remove it. I do like its application on shots that do not have a lot of sky, and it works especially well for water, where I do not want reflections. Out here in the desert, we have sun - waaay toooo much sun. I use the filter to eliminate some of the sun, rather than stop way down. Also, the polarization helps with the washing out of colors, under extremely bright sun. So, I can run at f8, which is usually the sweet spot for IQ in the lens. With the CPL, I can keep the filter, add polarization for the effect, and reduce the sunlight by a stop.
Wide angle lenses, in and of themselves, then to be very contrasty and have great color. So, when compared with other focal lengths, the WAs and UWAs will tend to have a more deep colorful images - even without the filters.
As to your original question, if you are going with just a single lens, then (and I am talking generalities here) ....
- Budget - Wide angle lenses are not cheap. They are more difficult to design and build, due to the complexities of having to bend light, in order to pull in a wider view. That is where the complexities come into play. The bending of the light creates the distortions - and the lens designs needs to mitigate them. The Tamron 10-24 is probably the most affordable, and it does a very good job.
- Physical Size - The lenses tend to be somewhat large. The DA*15 is the exception. You currently have a couple of large lenses, so I'm guessing that the size will not matter that much.
- Zoom vs Prime - Zooms offer more versatility. Primes offer better image quality. The DA 12-24 and the Sigma 12-24 have some pretty high image quality coming very close if not equaling the DA*15 in some ways. Wanting a single lens solution, I would suggest a zoom, just for the versatility.
- Distortion - This is picking up from where the budget discussion left off. The wider you go the more distortion you gain. Its just a matter of optical physics again. The fisheye effect is just the uncorrected effect. Rectilinear lenses on the other hand corrects the distortion. I'v found that a very good middle ground is the 12-24, 10-20 range. You are always going to get some, and these lenses tend to offer a very good balance.
- Focal Length - What focal length is best. As said earlier, the wider you go, the more distortion. For zooms, a good zoom range is 2x. Normally, for lenses - 4x is usually the norm (the ratio between the lower and higher focal length i.e., 50-200 is 200/50=4). Again, due to the optical design complexities, WAs and UWAs are usually 2x. In terms of primes, well they are by definition single focal lengths - 14, 15mm. Each mm wider you go, the Angle of View tends to increase. With the increased in AoV comes the increased in distortion. That is why I personally like the 12-24 range.
Here is a comparison of three of the most used WA lenses.
So, what would I do - or what have I done to address the perspective? Well, I like wide angles - to the extent that I have the 10-17 Fisheye, 8-16, 12-24, 16-45, 25, a couple of 28's, and the 31 Ltd. My suggestion for a single lens solution would be either the 12-24 or the 8-16, for several reasons, one of which are they are essentially the same price ~ $700 (no - not inexpensive)..
- If you are going to go wide - just go all the way - with the Sigma 8-16. It is wide - possibly too wide at times (at 8). But you always have the other end at 16 to bring it in. It is a well corrected lens for distortion. Its is still there, but better corrected that other lenses. It is contrasty and very colorful - can't use filters, but you really do not need them.
- All around useful - the 12-24. Not as wide as the 8-16, the 12-24 is a wonderful focal length range for everything (and its stitchable too). You have a couple of excellent choices here, Pentax, Sigma.
Now, there is yet another option - stitching. You can use any lens for landscapes (wide angle), and stitch them together. The benefits are that with longer focal lengths you get more detail, while removing the distortion inherent in wide angle lenses. Its really easy to do, and the software can be free. Take a look at Microsoft ICE.