There is not much difference in quality at the 20" size. You can get slightly better components on some bikes vs. others but in the end, the kids at 8 are not riding enough to warrant significant upgrades. I think the only justification for spending more on a bike for a kid that age is if they are the oddity and really bike a lot or they have siblings that will inherit the bike.
I'd recommend deciding if she is ready for gears or not. Her bike should at least have a hand brake backup if you go with a coaster brake. I would also recommend a bike with a rigid fork. Kids shocks for the most part are heavy and junk. Make sure the bike fits her (obviously you have done that part). That also includes crank length, handgrip size, reach to the bars and brake levers. Also make sure that they are able to adequately finger reach the lever and that the lever is not too stiff. Don't go for anything other than a 3 piece crank. Ride the bike yourself and check the pedals. I don't think I have ridden a single 12" or 16" and many 20" that have bent pedal axles. Sometimes, the shop can't even find a set of pedals that are not bent. Somewhat minor, but you are buying new and can expect some quality control for a $300+ cheaped out kids bike.
The weights of these bikes are pretty bad, often more than a decent adult road bike.
If you want to spend the money, Kona makes some nicer kids bikes. Heavy, but better made than most others. I really like the Islabikes out of the UK but they are not distributed here yet as far as I know. All else equal on the others in terms of fit, then I'd flip a coin or go where you'd rather spend your money. Of course if she likes it, it will probably be ridden more
If you can get a used one, I'd definitely recommend that route. No reason to spend alot on a bike that will likely see <1k miles of use.