One of my gun buddies has a SIG and so I get to shoot it. I like it and I would like to own one myself. However, I like to have one or two of everything.
At some point, a slightly superior design loses out if the other design is just "everywhere." I think that's the way it is in the US with the AR. Even Remington and S&W are making AR-15's now. Brownell's puts out a separate catalog with _just_ AR parts in it! (I've spent a lot of time going over it page by page.) As somebody already mentioned, the scales tilt even more toward the AR design if you have US military experience and trained with the M16.
The AR design is mature. Decades of maturing. Parts are everywhere and in a bewildering variety of types and manufacturers. And the whole upper/lower feature of the AR makes it super flexible. Since ATF says the lower is "the gun," you can buy or build as many other uppers as you want without an FFL getting involved. You can have a dedicated .22 LR upper. A 9mm/.357 SIG/10mm upper. 16" M4 style upper. A2 style upper. 24" varmint upper. 18" 6.8 SPC upper. 20" 6.5 Grendel upper. .458 SOCOM upper. On and on and on. Each with their own dedicated sighting system all zeroed for that specific upper. The AR-15 is the T/C Contender of the black rifle world! And with all the aftermarket parts, it is the Glock of the black rifle world as well!