I'm with Kirbinator
I've always believed its a good idea to have more than one gun identical, or very similar, to your primary carry gun. If you ever use the primary carry gun to defend yourself, police are going to take it for days, weeks, months or years. You may or may not ever get it back. The last thing you want is to have shot an assailant and be without a gun to protect yourself because yours is being held by police. Bad guys have family, friends and "business associates" who might not be very happy you've put a few holes in your assailant. They might even come looking for you to exact retribution. This is the worst time to be without the means to protect yourself. And if your primary gun is a semi-auto, you don't want to have to switch to a revolver. Life is going to be stressful enough without having to make such a radical switch in firearms. I'd prefer to have an exact, or almost exact, duplicate of the primary carry gun.
I usually carry a SIG P-229, so other classic SIG's are perfect. The P228 is almost identical in size, just 9mm instead of .40/.357 SIG. Fits the same holster. P-220 is a bit longer in the slide, but fits many of the same holsters. The P239, 245, 226 and 230/232 all have controls that are almost identical to one another.
I also have several holsters, in both left and right hand configuration. This turned out to be a really good idea. I broke the pinkie finger on my shooting (left) hand two summers ago. Nasty injury. The middle bone of the finger shattered into many pieces. Surgeon said there were more bone fragments on the x-ray than he could count. Treatment involved a surgically implanted, external appliance, so use of the left hand was out of the question for months. Fortunately, I just switched to the right hand holster and was back in business as soon as I was off the painkillers. I frequently practice both left and right handed shooting at the range, so the switch wasn't all that bad.
The other criteria is money. If you have money in the bank, you don't have enough guns, magazines, holsters and ammo.