You can't go wrong with either cartridge for a deer gun. However, the 243 is less recoil and usually less weight. Since you have the options to let her try both let her decide which is to her likeing. My fiance' shoots a Rem 700 ADL in 243 that I give her several years back she took two nice does 2 wks ago in KY one at 140yds and one a 75 yds. Both heart shots. Well place shots require a gun that fits the shooter and one that doesn't make them recoil shy to pull off the POA.
Hope this helps.