i know how you must feel. been there, done that. i finally made the weirdest choice you could imagine (not surprising, if you know me
): the zenitar fisheye 16/2.8. on aps-c it's a nice, distorted ultrawide. the distorsion, being a fisheye, is very regular and very easy to straighten out on aps-c. the image wuality is generally excellent. the price is good too. i had been thinking of getting an ultrawide zoom, once i have the finances (the 12-24 pentax is so unjustifiably expensive..), but after using the zenitar for a while, i just can't: it's small, it's compact, sturdy, it's perfect for my needs (outdoor/landscape mostly).
here's an example of the kind of stuff i do with it (to give you an idea, this is _not_ corrected -- can you tell?)
i guess from your list, the ltd 15 is the closest one, but it's not as wide as the zenitar (don't be fooled, 16mm fisheye is not the same as rectilinear, after straightening, this lens is closer to a 14mm rectilinear). i don't know how big it is and how it handles, either, the zenitar is tiny, and i like that a lot (that's because of the simpler fisheye design, rectilinear corrected lenses tend to always be bulkier for the same maximum aperture)
i also considered the tamron when it first came out, btw, for the price, but from what i read, tamron screwed up on the optical design with this one
. everybody has good things to say about the sigma 10-20 (and if you want proper ultra wide, it's hard to beat on price too). and last but not least, i've had my eye on the 14/2.8 from samyang: cheap, pretty good (though it has some distorsion), extremely sharp as it seems, and, like the zenitar, it can be a blast on film if you're bored one weekend
. i have the 85/1.4 from samyang, and i am quite impressed, they are to be reckoned with! but i'm not letting go of my zenitar any time soon, the combination of size, quality, etc is just perfect. i wish it was k-a instead of k-m (like the samyangs are)
good luck is all i can say (i know i didn't exactly make it easier for you
)