I'm fairly new to the AR world. I've had a Stag Arms 3G for about a year and half and really like it. I've decided that I "need" two AR-15s and was planning to buy a PredatAR and OBR 5.56 from Larue and keeping two of the three ARs I like best (or giving one to my son or daughter).

I want one AR for tactical shooting/defense, steel shooting (w/.22 conversion kit), plinking, and for my son and daughter to enjoy shooting (they are 11 and 12).
I want the second AR for longer range shooting with a scope for varmints and such (mainly prairie dogs). I am assuming that if I had a 3x magnifier behind my red-dot, that that would be insufficient for prairie dogs. I debated getting an AR-10 (for longer range shooting), but don’t see a need since I already have a 300WinMag (and .270) that I primarily use for hunting.

I reasoned (probably incorrectly) that I would want the tactical rifle to be light and short, so I ordered a PredatAR (lighter weight barrel that is accurate until it heats up). I ordered it with a 16" barrel.

I was going to order an OBR 5.56 at 18" for the longer range shooting.

I am now wondering if I made a mistake and should do the following:

Tactical shooting - OBR 5.56, 16" barrel. Weighs more. Shoots the same when dumping a magazine and heating up.
Varmints / Longer Range Shooting - PedatAR with 18" barrel. Weighs less for toting around. Typically won't dump magazines, so will shoot consistently. Might be easier for my boy to shoot because it weighs less.

Here are some questions for you experts:
1. Comments on my new line of thinking?
2. For varmints, would you go with an 18" barrel or a 20" barrel?
3. Is Larue a good selection in terms of moving to a higher end AR-15?
4. Is my assumption putting a 3X magnifier behind a red-dot is inadequate for prairie dogs a valid assumption?

Thanks in advance for your feedback!