I don't have the Sigma, but I do have the DA50, and the FA50 F1.4. Wide open the DA50 is a lot sweeter and more contrasty than the FA50. Plus it is lighter and very cheap indeed. I have seen examples of shots taken wide open and it is indeed even better. But when stopped down the Pentax pair overtake it in IQ (Look on Flickr)

Personally I would go the DA50 route, as bangs for buck, and more, it is a bargain.

There is another option if you don't mind slowing down a little and manual focus only. The SMC-M 50mm F1.7. Now that is a very good lens indeed and very cheap to buy.

Difficult to say, it all depends what the OP wants out of the lens. As he has said that he is considering getting the DA* it would appear that the price difference may not matter. Having said that, the advantage of the DA* over the siggy is that it is sharp wide open and with that you can separate the subject from the background. With that in mind and if you crop, you still get a great image from the DA*. But of course it also depends on how big you want to print the result and how much you crop from the image. I suppose that having sensors well above 10mp you have that comfort of cropping with a shorter focal length, yet still have excellent IQ. I am not dissing the Bigmas, on the contrary, they are very good lenses for the price and do produce great images. But then again It is not a surprise that a number of photographers I know out grow them due to the sharpness wide open or even lack of a fast aperture wide open, and have switched from the various bigmas (170-500, 150-500, 50-500) and gone down the route of 300mm F4's because of the faster aperture and ultimately bought a 1.4x converter in due course.
Only the OP can answer the question of what he wants out of the lens, whether he is happy with his Bigma, or having and ultimately a superior optic in the DA* and having to crop until adding a converter. Also it all depends on what he want to do with the images as well.

David has basically summed it up for me. Smaller, sharper wide open (albeit I am comparing the 300 with my 170-500 instead of the 150-500), superior bokeh, and better contrast in all light. I have had to work a lot more with my bigma than I have with the DA*. The DA*300 it just better out of the box.
Algi does make a point about the flexibility of zooms, hence why I have not got rid of the bigma, but for ultimate IQ, the DA*300 has it licked.

It is a very creative lens. I have had mine for years. Subject matter? Well Landscapes, churches, buildings. But then again you could give it a go with almost anything. Well apart from trying to get a kingfisher in the tree on the opposite side of the river, it would just be a speck...LOL

Best thing is with the lens is it actually gives you more ideas for a shot. You put it on the camera and you think, I wonder how this shot would work with my superwide. You play and come out with some images out of the norm. The same goes with the 10-17 Fisheye. I am lucky to have both, and I do have a lot of fun with both. Not always with great results, but then again, you try something with it and it turns out excellent.

I would say, just get one, and have fun. Throw away preconceptions about photography and just take photos with it, let all those creative juices out. You may find you are hooked.

Edit. Just noticed the line that you have got one.

Someone, sorry, a number have said what I was thinking, 15 and 55-300. Then again if you have room for one more, the 35 too, just for those natural focal length and shallow DOF shots that you may come across.

You are more than welcome mate

Good choice Mille19. I got mine from SRS a month ago before this wonderful offer. Gutted Still it is an excellent converter.
Now for the others, I must resist, 77mm F1.8 for less than £700, must resist......

I hate my photo being taken. Odd for a tog. But here we go.



Oh, hold on, that was when I was 8.

This might be better

Looks like an Alien from Doctor Who. Great capture John.

Shame the weather was poor during my trip, but I did manage to get a couple of waterfall shots.




The Aira shot was my K-01 and SMC-F 28mm F2.8 and the Crummock shot was the K-01 and Tamron 17-50mm F2.8