I've been using both a fair bit and I have trouble figuring out which one I like better.

12.7mm has massive sneak attack dmg with the right perks
12.7mm I've found is a bit harder to find, but with only two types to worry about, you don't have to consider swapping ammo types out too often.

Assault Carbine has better midrange accuracy
5mm Ammo is plentiful from some vendors, but it eats ammo like a minigun
Also even though the gun has 700HP it can still really chew it out.

Then there's the question about whether the assault carbine might recieve the suppresor, or if the 12.7mm Sub might get a stock to make it a tad more accurate....

Use both - Assault Carbine is far superior vs armored foes, while 12.7mm rips apart unarmored enemies much quicker. I'm currently using Fast Shot so neither one is very practical for me, but both are cool guns that I like.

Assault carbine is much better.

You don't get a whole lot of sneak attacks with the 12.7, realistically speaking, unless you're killing neutrals or friendlies. The gun's accuracy doesn't allow it, and that neutralizes the bonus from The Professional.

Assault Carbine, on the other hand, benefits from Commando, and can be used effectively at fairly long range

5mm ammo is probably the most common ammo type in the game, and can be bought and found in massive quantities everywhere. Large amounts of 12.7 ammo can be found at only two vendors, one of which (Contreras) never restocks; for me, anyway.

using an Assault Carbine in VATS with Commando/Sniper and Concentrated Fire is extremely deadly... and if your having problems with Brotherhood, Centurions or Rangers. Just put some Armor Piercing Rounds in it, and you have a -25DT FULLY AUTOMATIC That's Extremely effective in Close to Medium Range. I wouldn't be using the Carbine for long range, but for Urban fighting, or indoor fighting, besides a Riot shotgun with Slugs, the Assault Carbine with AP will chainsaw through enemies..

A riot shotgun with 'Stay Back'. End of story.

A Huntering Shotgun fully Upgraded, with Slugs, Shotgun Surgeon and Stay Back.

End Of Epilouge to aforementioned story.

The .45 auto machinegun has better spread and damage and clip than the 12.7mm submachinegun. The Assualt Carbine is a very good all around weapon.

^what if he doesn't have and DLC's

It's also not in the original game.