even half an hour is not enough... i'd say you need to test for at least 2 full sessions before you can make a comitment. Also it depends on the riders skills heaps.
from what i've seen (including personal experience), most people that switched from seat to waist harness never looked back (including me). I initially thought i'd keep my seat harness and use it every second session to let my back rest... but i never used it afterwards. In the end it all depends on her style of riding - whether she wants to push the limits, or just sit in a chair and go for a cruise.
speaking of different bits, if anything, girls should (generally) have less problems with waist harness riding up