A 100-gr bullet from 243. win goes at about 2950 FPS, a 150 grain bullet from a 308 goes at about 2500-2600 fps......
Now regarding barrel wear lol, its sooooo insignificant its not even a factor, you might be able to shoot a 308 barrel (with proper care) from 6000 up to 10,000 rounds, now if your comparing a 243 barrel (with proper care of course), you blast anywhere from (again) 6000-10,000.
THere is a new type of ammo available in the market, its pretty good, called Wolf Gold ammo, brass cased, full metal jacked = none of that bi-metal creap, its real copper from the outside and lead from the inside. VERY accurate on all my 30-06 rifles. That same brand also sells 243, 6.5 swedish, 7mm mag, 308 win, 300 mag, 30-06, and much more. Most of the time I buy a box at about $8-10 (depends on caliber).
I dunno how a shoulder can be hurt from 20 rnds of 308, it doesn't hit "that" hard, UNLESS your pulling the rifle's stock backwards toward you shoulder when you pull the trigger.
But seriously, if the 308 bruses you after 20 rounds, then MAYBE that should not be your choice.....You said it your self that you want a rifle that you can shoot and plink with.... to me plinking is not shooting 20 rounds and calling it quits. Plinking is shooting and shooting and shooting, for hours and hours a day (AND I LOVE IT LOL).
Get a 243 if you want to plink and MAYBE sometimes go deer hunting (like once a year or so)
Get a .223 or so if your only going to plink, very easy on the shoulder, and fun as all heck to shoot