The .223 is going to be easier on wallet and recoil less. I have to ask however if you are ever going to decide to hunt with it? It's very possible that you'll get the bug to go hunting in the future,and if you do-the .243 will be your best bet then. As to just target shooting,both are equally accurate at that range but .243 costs more to shoot.
The .243 will do everything the .223 will do at 150 yards and better, It retains more energy down range and Is more accurate, and has a much bigger ammo weight selection, most states dont let you hunt with the 223, the .243 is a game cartrige, the .223 is a varmit cartrige, If you are going to shoot junk surplus ammo get a M-44 or a 91/30 in 7.62x54 that will fill your 150 yard gap and plenty more for penny's on the dollar, if you want a great round with a little more flexability get the .243 good shooting ammo for it runs about a buck a round, premium ammo ( ballistic Tip ) about 35.00 a box of 20 rounds, remember ammo manufacturers have already done the math, you get what you pay for, and realalisticly just how much are you going to shoot it anyways, My goal when target practising is to get the tightest group possable, and you are not ever going to do that with cheap ammo, so buy the time you spend the money buying .223 top shelf ammo your just a few dollars away from getting a much more capable round like the .243
I totally Agree with Rick, listen to him. If target shooting at pop cans and watermelons etc is all you will be doing, then the .223 with cheap ammo might be fine, but if your gonna actually try to hit scoring targets and get good groups, then less shots with better ammo is the ticket. I love my .243 when I am with my friends and actually shooting for scores. I have a .223 but dont shoot at anything other than silouettes that go "ping" with it.
I was given the .223, And having had it for a while, I know I wouldnt have bought one now, knowing what I know.
I have used my .243 to take several trophy caribou at 300 yards+ with 100 grain bullets. You possibly will just end up selling the .223 when the hunting bug hits you, and if you shoot enough, it most certainly will.
All that surplus ammo is really just crap, I almost ruined my cousins AR-15 when one of the bullets fizzled instead of popped and a bullet lodged in the barrel. It now doesnt really shoot too well, even though the bullet was removed by an excellent gunsmith. That surplus ammo is unreliable and sometimes age catches up with the powders and primers making one semi-dud bullet ruin a perfectly good rifle. He still hasn't totally forgiven me, even though I gave him the pick of several of my rifles. (he took the browning BL-22 lever action .22cal which I loved!!!!!!!)
If your competeing with friends for 20 bucks or something, remember 1 thing, the .243 bullets weigh more on the spread, which means wind and gravity effect them less given they are moving the same speed.
they talk bout cheap ammo for 223 plinking cans and the 243 better for hole in a hole shots but you can still get good 223 that will do the same thing for less the half of the price of 243, hell.. even in my ar i can shoot the russian stuff and keep them in a quarter size group at 200 yards, with better ammo i can keep them in a dime size group easy