Pricewise there's not much difference between the cheapest 7.62x39 and the cheapest 223, $.25/round for Wolf, Brown Bear, et al. You have more variety of options with 223.
I've decided to send my DPMS Sportical upper to Rainier Arms for retrofit with a PWS piston kit. I figure since the Sportical upper is my least expensive AR upper it would make the best "beta" candidate for new technologies. My theory is that with a gas piston system it'll be easier to keep the receiver clean and cool while using the dirtiest cheapest Russian ammo available, and after 1600 rounds I'll break even.
Accually I didnt answer the Op's question so I will give it another run. 1 the only benefit of the 7.62x 39 round would be the cost of ammo, but what I like aabout 223 is that you can get some pretty cheap sstuff and you can also get ssome nice high end match stuff, its kinda hard to get ahold of match 7.62x39. I guesse 7.62x39 offers a better wound channel than 223 but again you can really find any quality ammo in the caliber. I think some heavy 556 (70+ grain) would serve you just as well as any X39 out there. If you are going to go that way I would suggest a 1-7 twist rifleing, accually I would suggest that in general unless you are useing it for light varmint rounds. I think I understan where your coming form you just arent digging the itty bitty bullet, you might consider the 6.8 remington SPC or the 6.5 Grendal, the SPC being the cheaper of the 2 alternatives.
Thanks you everyone for the info ... I figure when I build I'll go with a 5.56 configuration since it's what I am more familiar with ( spent 12 years in the army). Now, is anyone familiar with the gas piston operating system? Will it increase reliability and does it have any effect on accuracy?
The gas piston system is nice if you shoot a whole lot at one time due to less fouling. I haven't noticed any difference in accuracy. I had a 16 inch heavy barrel G. P. upper and got rid of it as more bother than it was worth. Like you I served in the Army (10.5 years and a med discharge) and can take care of a standard AR. I have 3 ARs 1 20 inch .223 varmint rifle, 1 16 inch heavy barrel predator hunting rifle and a 16 inch 6.8 SPC for larger game. All are direct gas op and as long as I do routine cleaning, I've never had a problem. I would point out the Armed Forces have not adopted for any use a G. P. AR platform. I believe this is because the G. P. system is a bit more fragile than a direct gas system.
It was a CMMG. While it was a bit cleaner than a direct gas system, tear down and cleaning was more involved and took at least twice as long. I had the M-4 LEP upper I don't recall if it was version 1 or 2.