I own the 14.4V impact, ratchet, and screwdriver. Works great. Paper spec is 120 foot pounds for the impact, not sure it puts out that much but i'd definitely say its working torque is around 100. The only real complaint I have is that the batteries are a pain to release from the tools. Snap-Ons warranty is also only 1 year which I think is appalling, and I've already had to warranty 1 battery. Also I think they should of made a boot for it considering its nearly $400 for the kit. Mines already pretty well beat up.
The DeWalt 12v MAX stuff is also nice, I have everything but the impact wrenches for home tasks. With the promo on Amazon the 3/8 impact kit can be had for $85 which is an awesome deal. Not sure how much the MAC flavored one differs but I see its impact housing looks lengthened compared to the DeWalt so it may very well be more powerful.
In terms of physical size they're about the same.
Some other differences worth noting
Snap-On uses a rocker trigger for direction, MAC has the standard button above the trigger
Snap-On batteries are 2.0Ah, MAC are 1.5Ah
Snap-On recharge time is 55 minutes, MAC is 40 minutes.
Snap-On is made in China, MAC is assembled in USA (I think*)
Snap-On warranty is 1yr on everything, MAC is 3 years on the impact and 2 on the batteries
Snap-On batteries are proprietary and costly, MAC will interchange with off the shelf DeWalt.
Snap-On is ~$380, MAC is $300