Sigma Prime Lens Advice?

With some monies kindly provided by relatives this Christmas I want to buy a fast prime lens for my Pentax K20D dSLR, which has an APS-C size sensor. The kit zoom lens is too slow & my old 50mm f1.7 is too long (because with the crop/zoom of the APS-C sensor it becomes ~75mm equivalent).

I'd love the Pentax 31mm f1.8, but that costs £1139 which is far too much! Instead I've been looking at the selection from Sigma, in particular the 24mm f1.8, the 28mm f1.8 & the 30mm f1.4 which are all between £300 & £430.

I think I've decided against the 24mm because I think the equivalent 37mm might be a bit too wide (it's also the most expensive of the three). That leaves the 28mm & the 30mm but despite having spent hours & hours looking at reviews & opinions on the web I can't find a definitive agreement on which is better. Both seem to suffer from quality control issues & whichever I chose I may end up sending it back a few times before I get a good one.

The 28mm is bigger & heavier because it works on both full-frame sensors & cropped sensors, though I'm not too bothered about that. Because of this, reviews generally say it has sharper corners than the 30mm on a cropped sensor. It focuses closer than the 30mm. It's also the cheaper of the two by about £70.

The 30mm is smaller & lighter because it only works on cropped sensors. It's marginally faster at f1.4 rather than f1.8 but I don't think that's really much difference. Reviews generally say it's rather soft in the corners at f1.4. It also has a HSM motor which the 28mm doesn't.

Can anybody provide any more input about either or both of these lenses? In terms of specifications they're both extremely similar, but I really want to make sure I buy the better of the two but because I don't live anywhere near a shop that stocks both for me to test I have to rely on opinions & reviews on the Internet!

Picture is just something I snapped on the kit zoom on the way home from the lab last week.