I have both the SW9VE & Px4 (9mm F build). The Sigma was my first pistol, bought for HD in December. It was reasonably priced, and I got 2 free 16-rd mags. I bought it for the stiff trigger (a sort of built-in safety), price, and S&W rep.
A few months later, w/the pistol bug biting me hard, I bought a gently used Px4. The ergo's & pointability are perfect for me, and it shoots like a dream. It has three different sized backstraps so you can fit the gun to your hand, and is DA/SA.
Over 1000+ rounds have been fired through both guns, and I've had zero hiccups w/either one using a variety of JHP & FMJ ammo.
Personally, I prefer the Px4: it fits my hand a bit better, is a better shooter, and is just downright sexy. But, even gently used, it was almost $100 more than the Sigma. The Storm is my bump-in-the-night piece, but the Sigma is still loaded & resides on the other side of my bed (should the GF or dog shove me to that side
). I'd have no qualms using either should SHTF. If you can find a gently used Px4, go for it - you won't be dissapointed - but the Sigma is a fine choice as well.