You would likely not be disappointed with either choice.
AS:
More nutrient rich so if you're sometimes less than consistent with ferts, AS provides a 'buffer' against that variation.
It is also reported to provide slighly better plant growth, and is surely easier to plant in because of its shape and lighter weight.
Light weight has disadvantage: virtually any modest water disturbance (e.g., refilling the thank with strainer too close to substrate) will blow it around. Some report more cloudiness from particulates, especially when you move things around, but in most cases cloudiness does not last.
Do not be concerned about KH suppresive effects, or ammonia. With initial fast growers, some zeolite, and decent water change schedule, you will be fine.
Flourite:
Solid choice, but not as easy to plant in, not as nutrient loaded. Pre-rinsing essential but not that big of a deal. Its only a big deal if you forget to do it the first time. Good strainer + garden hose.
I have used both. In the end I prefer the flourite because I got tired of "checking" the AS tank to see if sediment cloudiness had cleared after each water change or after doing replanting. Also didn't like AS pieces getting moved around by refill, even though I use a strainer and not at substrate level.
HTH
