You could watch your bullet with either of those cartridges.If I was in your shoes (I hope you wear 12) I think I would opt for the .243,You can still shoot 55gr through it also.In a target/varmint rifle they will weight enough to hold the bucking down.
I just cant help myself I have to say I really love my .220 swift,it has been alot of fun loading for and its hard to stop shooting it.I have enough space here to shoot out to 1000 meters. No I cant hit worth a $$$T at that range but its fun trying. Now with my .50bmg watch out