I've killed deer with both, mostly under 150 yds. 300 BLK performs better in my opinion. Usually a complete pass thru, nice exit wound. The one deer I shot with a 55gr .223 sp bullet exploded about the time it entered the neck. Still ended up with a dead deer but I haven't shot one with it since. If I think I'm going to be shooting past 150 or so at a deer I'll usually take my 7x57.
Concho "I know enough to post here in redneck pot smoking bunker......that's for sure."