Gotta tell you, if those are your only choices...hmm. 243 offers great trajectory and on the short end for energy at 300 from a short barrel. 7x30 is a great round, but not the best of trajectory or energy for 300 yards, I think both will get the job done IF you are a very good shot, use proper bullets for the task, can judge distance very well, but if you have other choices.... I would strongly recommend 7mm-08 or 260 Remington if you want to play at 300 yards. I used to shoot lots of sillywets and if you want to put something down for good at long range and for a pistol, heck even most rifle shooters, 300 yards is a long shot- you used a 7mm-08 or 6.5-08(260 Rem).
regards,
Graycg