Alright guys, I need your help.  

I am looking to get a carbine upper, (16 in, preban, flat top)  and I have kinda run into a small problem.

I cant decide which caliber to get.

So, I am asking for your help.
The choices:

.223 1 in 9 Chrome lined
.223 1 in 7 Chrome lined
6.8 Remington SPC
.300 Fireball

I am not currently set up for reloading, but I am in the process of getting in to it due to my other AR's love for 77 gr SMK's and 80 gr SMK's. . . .
Oh, and because I am a broke college student (and there are more than 2 choices), I can't afford to "Get Both".  

Thanks,
Krazny13


EDITED because I can't spell SMK

Do you already have an AR? If not, go for the 5.56mm first.

I'm going to get a .300 fireball next, as Model 1 sales barrels are inexpensive, and the cases/bullets are easy enough to buy or make. 6.8 would be great, but I'm hearing things about the military discontinuing the program, and it looks expensive, even for reloading.

5.56 1/7. You can use 55-80gr bullets.

Wow. a broke college guy looking for a "preban" upper...hmm those "preban" lowers aren't cheap. Student loan? just kidding

I'm in the same boat like you and can't figured out which way to go. 6.8 sounds promising but its future is still too"iffy" at this time. I hate the idea of having to hunt down the 6.8 rounds that I could be using for an actual hunt. If your just gonna go plinking or shoot little varmints, you can't go wrong with 5.56 (223).  Many on this forum are partial to the 1in 7 twist. I seriously doubt I will ever shoot the heavier grains and I will stick with mil spec grain for my plinker ...so 1 in 9 twist would fill the bill.

What are you planning using it for?  If its just plinking and target shooting, get a 223; they're cheap (both to shoot or reload if you finally go that way) and you can find it anywhere if you forget your ammo at home.  
I dont have any practical experience with the 6.8 or the Fireball, though I guess they should both be good rounds for larger game.  Guy who runs a gunshop I go to says he shot an elk with the 300 FB this year using 220 grain handloads.  
I think its six one way, half a dozen the next between those two.  You may have a little more versatility with a 30 cal but I dont know if its enough to sway you one way or another.

I happened across a preban lower at a garage sale.  
And I have a whole stack of stripped lowers.  I think there be somethin goin on between my Preban and my DCM gun, cause everytime I look, there is another reciever sitting there. . .

Anyway, I do have a .223 AR already, it is a Eagle Arms lower with a Jewell and a WOP upper.  Built the lower myself with the ARFCOM instructions.  The Preban lower is sitting in my safe with a collapsable stock, and I have been leery of using my match gun as a blaster 'cause match barrels are expensive and short lived.

I would also eventually like to get a suppressor, but that is a long ways off.    

I guess it boils down to either:

.223 1 in 7 Chrome lined
or
.300 Fireball

I don't have time to go plinking as much as I use to and would like to carry my AR-15 while deer hunting, so I took the gamble and sold my 5.56 Armalite upper and bought a 6.8spc from Model1Sales.  
I believe that ammo will be readily available someday

I really like the ballistics of the 6.8 but I went ahead and built me a 5.56 cal AR-15. I want to build another in the 6.8 cal but I'm holding off until they decide what they are gonna do with it. Besides all I'm doing with my AR for now is killin a bunch of clay pigeons n paper targets with it. And ammo is cheap too so thats a plus.