Mario - the big thing to watch out for when you go to .308 in a AR platform is compatibility. For what I've seen, there are 4 different patterns used with Armalite and DPMS as the most common (FYI Noveske and Aeroprecision are the Armalite pattern). All the patterns are different and will not exchange - this applies to more than the internal parts...i.e. a DPMS barrel extension is different from Armalite and different from a Stoner, you can not swap barrels across the different uppers as a result. Magazines are different too - Armalite is based on the M-14 magazine - DPMS is more along the lines of an AR-15 (MagPul makes DPMS compatible mags, but not Armalite). The barrel nut threads are also different, making the attachment of the forestocks incompatible. The bottom line is to stick with whatever pattern you choose in a AR .308 gun. This is a huge difference from the AR-15 world in that AR-15's are pretty much standardized across manufacturers.
The AR-10 is an Armalite trademark - the only gun you can call an AR-10 is therefore Armalite - it does have a majority of it's parts common to the AR-15 (this is why I chose it for my builds). All the AR .308 guns are more expensive than an AR-15, both for a complete gun or for parts - availability of parts is also a lot less and the compatibility issue is a real player. If you are going with a basic gun, buying factory is usually cheaper - if you are planning to do a lot of customization, a build will be cheaper since you are not buying parts you don't need. I went with the latter route on the two AR-10 guns I posted here and used an Armalite Gunsmithing Kit:
http://dsgarms.com/ProductInfo/ARMGSK10A4.aspx
Of course, the tools you need to assemble an AR-10 are different from an AR-15 too
The additional parts to complete add up fast, especially if you use top quality parts. That is why the AR-10 SPR I built and posted a few pages back is so expensive - there are very few stock parts in that gun...the barrel alone was pushing $600. The result is a gun that prints sub-MOA - it's groups are less than a quarter at 100 yards with a middle of the road scope and it's not even broke in yet.
It all depends on your objective - kinda like a Supra in this respect - do you want accurate, reliable or cheap, you get to pick two