First I will deal with your choices to date, then offer my response:
tarus 24/7- decent gun, but not an established track record of quality (though they do have a lifetime warranty) Tough to find holsters and other OEM items.
S&W Sigma, this is the step, bastard red headed child of S&W behind the SW99 and the new M& P series. Early models were universally considered to be POS. Newer models have a better reputation. More than one PD went to them because they were the same company that made their revolvers when PDs started changing over to pistols.
Many of those same departments soon went to Glock, Sig and other weapons.
Sig 225/P6. A very reliable and well built weapon though extra mags are hard to come buy with the latest release of German Police turn ins. My opinion, it is also harder to shoot a weapon well when you have to learn to manage two distinct trigger pulls i.e the long 9-14 lb first double action pull, followed by a short 4 -8lb single action pull. This is compounded by the remembering to use the decocker which lowers the hammer on the sig as well.
If I were looking at a first pistol, I would be looking very hard at one of three
1. They are all polymer lowered
2. They all have triggers that feel the same every time you pull the trigger
3. They all have reviewed well by multiple end users
A. Glock 19
B. Springfield XD
C. Smith and Wesson M&P.
In the interest of full disclosure I am very Glock oriented as my prior posts have indicated. Having said that there was no XD or M&P on the market when I started shooting pistols. I think the xd has issues re the grip safety (it must be fully depressed to run the slide)and the lack of available factory parts. Having said that,I know many people who switched from glock toxd because the xd feels very good in the hand. Many hard core 1911 shooters think if you are going to shoot a plastic gun, the xd is the one given how it sits in the hand relative to a 1911. XD mags fall free are are widely available. Moreover the xd in .45 is absolutely slimmer than a glock.45.
If I were a newbie today, I would give a real hard look at the M&P because:
i. It was originally engineered in .40 caliber such that it should stand up to the hottest of 9mm ammo all day long
ii. It has three diffrent sized removeable backstraps to fit a wide variety of hands
iii. It has a metal magazine which easily falls free.
I would make sure I got one that did not have the key lock or the magazine safety.