I would say you both have some great choices out there - and you have named some good ones - and you will likely experience a supply chain problem no matter where you look.
Three questions you should ask yourself:
1) Is this a tool or a masterpiece?
2) Do you want a military type (or even clone correct) rifle, or a great shooting rifle?
3) How are you going to use this rifle, and how far would you typically shoot?
okay 4 questions
4) What are the neighbors to this gun like in your safe?
These are important questions. They seem simple. But, with the "AR15 realm" you are dealing with rifles are all a their core the same. A $699 S&W Saint functions the same way a $2,000+ Barrelt RC7 does. So, how do you differentiate. Everyone here has their favorite brands, so you will get that.
My business is that of selling, assembling and making military correct rifles, so even if you want a simple military AR, I will probably point you to a mid-price point FN or Colt, and you will be happy. I also love some of the more refined precision rifles, both military spec and commercial. Something just feels good shooting a LaRue or a JP or a Barrett. It is like the steak at Morton's. Then again, the SIg Tread does a good enough job, if that is what you want. Sold, reliable, no frills. Think Outback Steakhouse.
So, back it up. With an AR platform, the most important part is the barrel. I think with a bolt gun, it is the receiver first, and then the barrel. But AR15 upper receivers are all close to the same, one quality brand to another. Yes, there are some real stars, and yes, there are some real dud, but inside the realm of general quality, the AR upper receiver is not your first worry. The barrel will determine mission. A chrome lined barrel is best for a military general rifle, and you will be well served with a 1-1/2 MOA rifle with a chrome lined buttoned broached or CHF barrel. An average sized barrel is 16". If you want short barrel, CQBR type, then 10.3", 10.5", 11.5" are what you want. Military M4A1 is 14.5". If you want a traditional old-fashioned AR15, like the Colt Sporter, then 20" rifle length gas system is your choice. If you are looking for accuracy, you now move to 18" to 22", with 20" being the norm, and normally stainless steel (416R) and preferably single cut rifling. Just like you would with a bolt gun. Now, you have moved from a kick the doors down to a general purpose rifle to a precision rifle. The first military true sniper rifle in 5.56mm (I am going to get some Vietnam guys argue with me here) was the Mk12 SPR, at 18" Douglas SS cut rifled barrel, in 2000. That gun changed thinking about the AR platform from a 1-2 MOA shooter to a 3/4 MOA shooter.
Once you pick the mission, you have picked the barrel, now pick military correct or best shooting. Certainly, there is some overlap. LMT, Knights and Barrett do both, and do both quite well. LaRue is also in that group, but has not seen quite as many military deployments.
Personally, I think scope comes after barrel, but let's put that aside. You can come back later.
Now, the upper receiver and bolt carrier group matter after barrel. The truth is, there are very few bad BCGs and bad upper receivers, within the middle 80% of AR15s on the market, but they do exist. Avoid the no-names. You can also spend a great deal on a BCG. LMT probably has the best performing BCG, and others, like Young's Manufacturing, Geissele, Surefire, JP all have their adaptation, and others that are not coming to mind. The difference between a solid BCG and a really outstanding BCG might be 3% to 9% improvement in performance or life expectancy and 2x to 5x the difference in price. Same with the upper receiver.
Next in line is the trigger group and lower parts kit. I will go out on a limb and say the lower receiver -- other than a solid buffer and buffer tube -- is the least important part. Even the trigger, is a personal preference, and you will need to decide: single stage or double stage. There are a handful of great trigger makers in double stage. Single stage, I will probably opt for simple. Double stage, I like Geissele and LaRue, but LMT, KAC are others are out there, and you can upgrade that later as well.
So, have I done a good job of muddling the waters for you. Go back to the basic questions, and answer those, and I can probably limit your search, and others will have equally valuable opinions.
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