I would buy a Springfield Armory Model XD-40 S&W. I own one for self defense use & carry one on the job. Springfield Armory makes a nice pistol for the dollar. You should at least look at the Springfield Armory XD, know matter what you buy. I think after you look at it you will like it?
Let us know what you buy.

These are just my opinions on a few different guns
S&W sigma is a decent gun, good DA trigger, but I didn't really like it for overall shootability. I had an older model, maybe they've improved them.
Taurus millenium, is a very nice shooting gun for what its meant to be. The 24/7 are supposed to have the same trigger mechanism. Its loooong, but smooth.
My beretta 96, is one of the most ACCURATE pistol I have ever shot. Its wild how good this gun is.
Springfield XD's are sweet guns with an excellent trigger.
A nice slick 1911 can sometimes be found at a good deal used.
I have no comment about glocks I have very little experience with them.

Save up a little more and buy either a Sig or a Glock.The Ruger "P" series are fine guns.If the Ruger is within your budget I would say go for it.I've never owned a S&W Sigma but have heard mixed opinions.For my carry gun I prefer my Glock M-27.It's fun to shoot and easy to conceal.There are so many choices out there.I would see if you can shoot different guns and see which one fits you needs.
Ruger Redhawk

If you are really looking at an automatic, you might want to save up just a little more money and get a Kimber. They make a great pistol with lots of extras at an affordable price (compared to what you have to pay for a custom pistol with the same features).

But, if you are possibly still considering a revolver, I would very highly recommend a S&W model 66 (.357 magnum). It is a very sweet shooting gun and you have a tremendous variety of ammunition you can choose from. Shoot .38 Special wadcutters at the range for $6.50-7.00 a box (shop around), and load up full house magnums for defense. These gun hold their value very well, and if you look around you can probably find a good used one in the barrell length you prefer. I have one from the S&W performance shop, and it is my favorite gun to shoot these days.

Best of luck whatever you decide! This is not an easy choice.

A lot of good guns suggested above, myself I'm a Glock & SA XD fan for the .40S&W.

My advice is to hit a range and rent some of the options given above by the members and see which one you like the feel of, which ones feel comfortable to shoot and which one you like. Just don't consider the accuracy and reliabilty of a range gun, they are shot alot and cleaned very little usally.

THat is really a broad question. I would select a gun based on ammo cost if I planned to fire it at a range a lot. I have a .22 revolver for plinking, then I have a .380 for defense.