Hey there, I use the flourish line as well.
My understanding from my LFS is that while both are from Seachem, the aquavitro line is available only via certain LFS (not online) and is more concentrated, so less volume to use in a tank. For example flourish nitrogen you can dose 2.5 cc (1/2 capful) per 40 gal, with aquavitro Synthesis it reads 7cc per 250 gal (from the website aquavitro.com). Or put another way, flourish N has a concentration of 15,000 mg/L while aquavitro is 32,000 mg/L.
Hopefully, someone else can chime in on how that compares to the dry ferts that many folks use on this forum, since I haven't used them. My guess is cost. You can tweak your dry ferts as needed without having to invest ~12$ for a new 500 cc bottle of flourish Nitrogen [vs 3$ of 1lb of KNO3 (from greenleafaquariums.com)].
Hope this helps and in the end, as long as you find a routine that fits your life and your tank's needs, you're all good!
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