The Navy SEALS and other miiitary units are using .300 Blackout.
300 blk has been adopted by to many SRT and SWAT teams
And I could assert that even more have adopted other calibers. The majority don't have that kind of budget money, tho, and paying double for commercial ammo isn't a big qualifier for the money maven at the office level. The .Gov handout rifles aren't getting modified because they are the property of Uncle Sam and subject to return.
Let's not make unsubstantiated claims about the .300BO, it HAS been around as the .300 Whisper for decades and just because Remington got behind it as the required accessory for their suppressor sales hasn't justified it for the budget mongers at the local level.
.44 or .300BO? Not only two different ballistic curves, but two different guns shooting it. Goes to the OP being conflicted over which GUN he would like to buy, not the cartridge.
At best, we could then discuss "which is better, the Ruger or AR15?" Do we really need to go over that ground again?
The issue is that the OP needs to specify exactly what his range and target is going to be for the rifle, not jump ahead to the cartridge - especially where the two are significantly divergent not only in ballistics but the type of gun shooting it.
Of course, it could then boil down that neither were the optimal solution, and something entirely different would be the better choice. Risky, that. But it would deliver better results, which is the point.