Seems like no one has the 223 in stock right now and I think 300blk is kind of pointless in Canada.
If you're buying premade cartridges then yes. 223 makes more sense; but "pointless in Canada"..
One can load from 90gr to 240gr projectiles. The supersonic loads will peak around 1500lb/ft energy VS 16" 223 @ 1060lb/ft.
A 30cal hole obviously bleeds out as it punches a bigger hole than a 22cal hole.
The brass is same price and plentifully available as its formed with strictly dies and some labour from prevalent 223 brass.
Primers, same.
Powder, the 16gr powder will push a 150gr projectile plenty well, vs 27gr I use for 55gr 223 projectile. So savings there.
Projectiles, bulk 223 55gr @ $120/1000pcs? Bulk 300 m80 ball @ $280/1000pcs.
Subsonics are useful in a 30cal, putting out more energy than 45acp does point blank and does so out to hundreds of yards.
You're neutering your 223 to 22lr energy levels by making subsonics.
223 is a great cartridge though. Don't get me wrong.
In summation; ballpark 40% more energy (if wanted), also works well in short bbl AR. I'm sold.