I'm at the beginning stages of my first ever build. I have a Stag lower, RR lower parts pack, VLTOR stock and Hogue Grip. I'll be adding a complete upper/barrel/receiver in the future.

(I'm ASSUMING that by buying a "complete" upper/receiver/barrel package I eliminate concerns about headspace, etc.?!?)

In the meantime, I've also bought an EO 512 and a 6-24X scope.

I'm debating a 20" "all purpose" barrel... with interchangeable mount for the EO and the scope. If I go that route, is there a mount-system that would give me the best luck at not having to do too much "re-zeroing" as I change between the two optic systems?!?

OR... the alternative would be to mount the 6-24X onto one upper with 24" barrel, and the EOtech on a different upper with a 16" barrel.

I'm trying to build the "one-platform-does-it-all" rifle, knowing full well I'll probably build 2 or 3 different AR15's eventually for different purposes and use different optics on the different platforms. I guess I'm thinking initially a SHTF rifle with EOtech but with the OPTION of playing around with longer-range shooting for fun with the 6X24X.

In the meantime- any thoughts? Suggestions? Input? Recommendations?

Thanks in advance.  

Ahh questions I can answer...

1.  Your lower parts are a good choice.  I have something similar on one of mine.

2.  You are correct.  No head space issues with factory builds (unless they screw up).

3. ARMS SIR or GG&G have great mounts that do not require re-zeroing.  I know because I use them both.

4.  I like you thinking  on the ARs.  I started with a 20" bushmaster from the factory that I modified over the years.  Then I added a 16" A2 commando.  Then a 14+2" M4gery A3.  Then a 223 pistol.  Then a 9mm AR pistol.  Then I bought another 20" and sold it.  Then I build my brother a 20" for his birthday.  Now I am working on a 458 socom carbine.  It's a sickness you know...

I forgot to add.  There are many rail systems you can add to avoid re-zeroing.

Samson, Troy, Midwest Industries, ARMS SIR, VLTOR, Yankee Hill, and the list goes on.  

Larue has a great mount for the Eothingy that raises the sight and allows for a lower third cowitness. Larue
GG&G makes the accucam as a replacement mounting solution for the EO's mounting system that I believe will retain zero. Yhm also makes an inexpensive riser that is in this same post.
GG&G and YHM
If it were my money I'd opt for the Larue. Great company, great people, great products, Period!