I've been laboring over the same problem for the last year too. I wanted something better and more accurate than a drop in 22 conversion even though they are cheap. The dedicated 22LR upper seemed like the right way to go but that costs as much as one of the "AR-22LR" complete guns.
Of the three I like the heft of the Colt M4 Carbine 22LR better than the S&W and both of them better than the dressed up Ruger 10/22-SR22. I've never handled the SIG.
The S&W probably is the smart choice, it's cheaper, made in USA and the internals (especially fcg) are more like a real AR. But I hate its look, weight and flimsy feeling of polymers - and all the missing features that make it scream "I'm an AR!" The rails are really chinzy, no A2 sight, no flash hider.
I just picked up the Colt M4 on sale at Sportsmans Warehouse and love it. It weighs about the same as a true M4 and looks an awful lot like it. Externally it's hard to tell the difference until you pick it up. Sure the outside bolt catch and forward assist don't work and the dust cover doesn't always work but at least they're there (along with a carry handle, F-marked FSB, castle nut and A1 style flash hider) and make this a near twin in looks if not function. I kept telling myself I was making a mistake going with my heart and against the S&W but now I know I made the right choice. Great little rifle.