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Accuracy has very little to do with barrel length. Shorter barrels will actually be more accurate, all else being equal.

7.5" won't give you any fragmentation range. Use it for a range toy or for home defense.
10.5-12.5" will give you 50-100m frag range, depending on the quality of the barrel.

You decide what's more important to you. If you need it as small as possible, but still want it to frag out to 30-50m, a 10.5" will work. If you want a little more frag range, go a little longer.

Keep in mind that most reflex suppressors won't fit on anything shorter than 12" barrels.

Obviously, a 7.5" will do a lot of damage out to 100m, but so will any .22 caliber rifle. If your property is 100m or bigger in any dimension and you plan of defending it with your AR pistol, get 11.5" or 12.5" for that extra power.

If you don't plan on using it for defense, get whatever looks cool to you or whatever is cheaper.

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Cool, thanks man.
Yes, HD. No suppressor for it. Id like to have one, but I dont want to spend the bread on one,  yet
Also, (me sounding completly dumb) what do you mean by frag, out of a 7-1/2" barrel it wont break-up or collaspe or tumble? (or somethin like that)
And a 7-1/2 inch will be accurate and "usefull" only out to 100 yards?

Im askin alot about the 7-1/2" I think I like that shorty in the pic thread

frag/tumble has little to do directly with barrel length, but everything to do with projectile velocity, the round out of a short barrel will have less velocity, therefor, will be going too slow to fragment at the same ranges as faster rounds(generally from longer barrels)

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So, firing the same exact type of round (like a reminington exp. .223 55gr pointed soft point) out of my 7" barrel, will be moving at a slower speed then out of a 16" barrel?

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Think of it this way: the bullet is accelerating the entire time it is in the barrel.  So the longer the barrel, the greater the amount of time to accelerate.  Once the bullet is out of the barrel it will start to slow down.  

As stated, most FMJ bullets tend to break up or tumble more reliably at higher velocities.  For me, I would prefer to use some type of expanding bullet such as those used for varmint hunting, as these will fragment at much lower velocities.  

Then again, back to your original question, I would consider your primary purpose for the gun.  If you are mostly going to punch paper or shoot gophers, get whichever barrel length suits your fancy.  If you are thinking of using a short AR for home defense, you might try shooting one indoors with no hearing protection some time.  You could very likely blow out your eardrums and see spots, much like when you hit your head hard.  Not the best option when trying to defend yourself.  Most guns are loud to begin with.  When you shoot them in confined spaces (like a hallway in a typical home) they seem much louder.  Enough so as to be temporarily debilitating to the shooter as well as to the intruder.  

Just something else to consider.