Electrical engineer here.
I would like to start with a 556 with an 18” barrel.
I would recommend 16" in 556. You get a little extra velocity out of the 18", but it doesn't add much usability. I prefer 14.5" the most, but due to laws it must be pinned and welded, and it's probably not worth tying yourself to a particular muzzle device at this moment.
I don't fully understand the differences between the models, and if those differences would matter to me.
preda(tAR) preda(tOBR), OBR All will shoot sub MOA. Practically speaking, one is not more accurate than the other. The design of the OBR, however, with its continuous top rail lends itself to a wider range of ideal configurations for long range precision shooting. The others are pretty much equally capable, but require picking the appropriate scope and mount configuration within the confines of the upper receiver rail whereas the OBR with the continuous rail doesn't care if you use rings or a mount.
I spent a lot of time searching for reviews that compared the three. I would have loved to find a review that provided reasoning of choosing one over the other. I didn’t find this review. So…
Generally speaking, tAR = light weight configuration, tOBR = medium weight (reduced upper mass, but same barrel as OBR) with break down capability, OBR = heavier than tOBR due to additional upper receiver mass.
Would one model be better suited for long range, or tactical shooting, etc.? Why?
tOBR and OBR, mainly due to weight, for long range. Best tactical shooting configuration... would be a tAR with a heavy barrel, in my opinion, but that's not currently offered, so I'd have to go with the tOBR there between the three. Additionally, the tOBR will allow you to drop in a caliber actually suited to long range shooting vs 223/556
Does the low profile barrel of the tAR heat up & change accuracy more than the heavy? If so, at what rate of fired does this happen?
It heats up quicker, but also cools down quicker. Within the range of 223/556, it's a non issue IMO.
Would the LT upper kits fit onto the lower of each model?
Was there a reason why you chose your model?
14.5" 556 tOBR - reasons stated above incl. break down capability, set up with red dot for 0-250yds for matches and general carbine type stuff.
+18" 6.5 Grendel barrel for tOBR 0-750yds(low/medium/high powered scope).
16" 308 tAR w/ heavy barrel - reasons stated above (also incl. easy break down capability), set up for 0-250yds(red dot) or 0-750yds(low/medium/high powered scope).
+22" 260Rem heavy barrel for tAR - long range configuration, set up for 0-1200yards (high powered scope).
The 18" Grendel barrel for the tOBR came from the ultimate upper kit, the ultimate upper will get a 12.5" 6.5 Grendel barrel
Does it feel dirty to order the LT upper assembly & put on a non Larue lower? Mixing them might drive me crazy
Based on what you have stated thus far, I think you'd appreciate a full rifle rather than mixing and matching.
Given that, for what you're looking to do, an ideal configuration would be a 14.5" or 16" tOBR with a RDS or Low power scope (e.g., 1-4 to 1-8x) and a 6.5 Grendel ultimate upper kit with a 4-16 scope. Just swap the two uppers between the lower depending on what you want to do, and eventually build out a 2nd lower for the ultimate upper if desired.
Thanks in advance for your help