First of all, the Jack Carbine's were sold out within a few days once BCM started accepting orders, so good luck finding one.
Quoted:
My other option is KAC sr-15 for 2,100. Do you think that's a better choice than BCM?
Second of all, KAC vs BCM is kind of like Camaro vs Mustang or Porsche vs Ferrari:
-There are lots of fans on both sides of the debate
-One may have a slightly better/longer recorded history of success, but it's like comparing Kobe's Lakers to Jordan/s Bulls.
-Either will probably run laps around a Bushmaster/Oly/ Ford Fiesta
-A good driver will have a favorite, but would happily use either and make you look like a noob.
BCM rifles are great and hard to get in normal times, good luck getting one anytime this year. KAC SR-15 Carbine wait times are running ~90 days right now I think. You will not be disappointed with a KAC. Once you get your hands on a KAC, you'll want another one (no homo).
[Full Disclosure: I have a BCM Mid on a Spikes lower and a KAC LPR. The LPR is an SR15 Mod 1 with a Kreiger barrel.]