The parent case of the .270 is the .30-06, necked down. The parent case of the .243 is the .308 Winchester, necked down. Aside from the larger diameter of the .270, the .270 has a longer case.
Size of the rifle will vary, depending on make/ model. The 30-30 BULLET is larger in diameter and weight than the .243, but has less energy. This is due to a slower bullet.
That depends. If you are planning on killing rodents then the airgun is better. But if you are planning on shooting other people then the air-soft gun is better. -Jeff