As far as I can tell, the main difference between USAG and MDL at the lower levels is in MDL there is no compulsory round. Our gym has the girls do MDL until they get to USAG level 7, then they switch to USAG.
My daughter is in the middle of her MDL years and practices 3 times per week at 3.5 hours a pop. My other competitive athlete, who is younger, also pratices three times a week, but those practices right now are 1.5 hours each. At the moment, gymnastics is a much bigger commitment time-wise, the meets are farther away, and it is more expensive than my younger child's sport. But the gymnastics kid loves gymnastics and the younger kid loves his sport, so I would never try to steer one towards the other. Our kids picked their sports themselves.
If your kid wants to do it, then you do it if you can. But be aware, too, the more hard core gyms also carry risks. One of our cousins had to bench her daughter for the season with many tears all around. She was competing at the USAG 5 level and, because of the rigorous practice schedule at that gym, was at serious risk for a stress fracture of the spine.
Same for our gym... the basic difference is MDL not compulsory at the lower levels (as mentioned) and MDL has 4 levels (A,B,C,D) that span USAG levels (4,5,6). [To note there has be a recent change in the USAG levels system, so my comments are in regard to the last year’s system. ] Our gym typically has the girls do their qualifiers in USAG Lvl 5 and 6 after level MDL Lvl D, but some girls opt to do it earlier. It is interesting to see the girls from the different gyms in MDL perform the various level skills and thankfully different music for floor ex. Our DD is in 6th grade and is at the gym is 3 days a week for MDL Lvl C and it goes to 4 days for Lvl D next year. She has a great group of friends on the team who are in 5th/6th grade from all over MD/DC/VA. Some girls are in at a very high level in USAG at younger ages, 3rd and 4th graders that are USAG Lvl 6 and 7 or higher– those are Elite track gymnasts will be trying for the Olympic qualifiers. But, then again they’re practicing a lot more…more akin to very advanced ballet students at a similar age.
As others have said gymnasts generally tend to be very focused, advanced academically and pretty balanced. MDL, for our family, has allowed DD to do some extracurriclars at school and gymnastics and still maintain her grades. We also have taken her to some high school meets and she sees that she already has more skills that some of the girls on her future high school’s team. She also takes amusing pride at being the strongest kid in her class (boy or girl) at President’s physical fitness tests, which kinda embarrasses the “jock” boys...and, yes, by 6th grade you have them.
As far as gyms go, I won’t get into a this’s and that’s about which gym, but a lot of gyms in Maryland to have MDL [ see MDL website here: www.masondixongymnastics.com ]. Some gyms have both USAG and MDL for students at the same age, others have MDL and transition to USAG at Level 7. I would check out the gyms that would be an easy trip distance-wise, has clear rules and whose coaching approach you agree. One thing we like about our gym is the focus on conditioning first and the balance of building up skills. Also, at least for our competitive level the parents aren’t too crazy

and get along well.