I have a 3x9x40 on my .243...Groups at 300 yards run between 3-4 inches...
With a 4x12 you get a longer, heavier scope, less eye relief at the higher powers and they are not as bright...I feel that a 3x9 is best all around...If you were going to get into say, groundhogs at 400 yards, maybe a 4x12 would be the ticket...Remember 40 years ago when a 4x was a good all round scope...shots were made at 300 yards with them also..