IMO I would stick with the .243 for your niece especially if you have a .308 already. The .243 is a very versatile gun with loaded ammo anywhere from 55grs to 105grs. The Howa rifles are a good well built rifle, a friend has one in 22-250 and is extremely accurate. The 7mm-08 and the .308 really have about the same recoil if your using 140grs in the 7-08 and 150grs on the .308. It's quite frankly hard to tell the difference in the two. If the over all weight of the guns are quite a bit different or your using 180gr bullet in the .308 then you can tell a difference otherwise not really. They both although still mild in recoil compared to others do have quite a bit more recoil than the .243. The Winchester 95gr XP3, a well bonded 100gr bullet or even the Nosler 90gr E-Tip ( Nosler's lead free ) would all be good choices for the .243. Good luck