I competed USAIGC about 6-7 years ago, so things could be much different now, but I'll let you know my thoughts.
I found USAIGC to be MUCH more lenient. Scores were higher, girls with missing requirements never seemed to be penalized much, any more than 3 falls on beam were not counted, it just seemed like a more self esteem building atmosphere. It was pretty easy to qualify to the National meet which was pretty exciting for most, I never got to go though so no comments on that aspect of USAIGC. I would perform routines with all of the requirements and score lower than my teammates who were missing key elements.
USAG is much more competitive, they really sock it to ya if you are missing requirements, scores for similar routines seem lower, and there is less flexibility.
As an example, I competed as a USAIGC silver and regularly scored in the 9-9.5 range on bars. I then switched gyms, they pushed me back to level 6 and I scored in the low 8s for a routine with very similar skills. I'm sure some girls who do well in USAIGC also score very well in USAG meets, but to me the scoring seemed much more lenient.
As a very competitive person who wanted to be the best gymnast I could be, I did not like competing USAIGC. It was fun for getting high scores, but sometimes I wonder if I could have been a better gymnast had I stuck with USAG competition. It enabled me to experiment more with skills and compete harder skills than my level required when I was ready, but it was just more "fun".
I know some gyms compete both, so those gyms might have a different approach to the program than mine did. I think it definitely has it's benefits, for kids who are in competitive gymnastics more for the fun of it or younger kids who are just starting competing, but for serious competitive kids I think USAG or at least a combination of the two is the way to go.
I hope this helps!
To know which one is best for your gymnast you need to look at her individual situation. USAG is a better preperation, for top gymnastics. If your gymnast aspires to compete at the elite level, or to compete at the college level, or even be a top competitors in the high optional levels then she will do better in USAG. She needs to be in a stricter competition environment amongst the most competitive gymnasts, in order to really get to where she needs to be.
USAIG is better for those who have a perticular difficulty which will affect their progress later on, example being a late starter, having a lot of fear, struggling with flexibility, training low hours and so on.