Yes, they're both well known brands.

yes and yes, just do some research on wich of them you are planning to buy and how you are going to use them. I suggest before buying anything, to always google "*name of the flavor and the fabricant + reddit"

Both are widely used. Both have some excellent flavors, some mediocre flavors, and some horrid flavors, as does every commonly-used flavor manufacturer.

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Also note that TPA (The Perfumer's Apprentice) and TFA (The Flavor Apprentice) are the same company.

Well, other than the fact some FW flavors are toxic/carcinogenic, either of them can be used. They aren't the best example of every flavor, but some of each are actually the best example of some flavor (like DX Peanut Butter, or Honeysuckle) so most have several of theirs.

You can visit established US stores for recommended vendors and flavors. If they are selling the vendor is good. Try these - nicotineriver, bullcityflavors and ecigexpress . These sites will also have flavor reviews which may or may not be useful.

Be careful though because some of these vendors make flavors that are safe to eat but not to vape. As pesopluma suggested once you find flavors look them up on google or reddit.

The youtube channel diyordie that has a program called "noted" that discusses flavors from different vendors. I found this to be really useful in picking out flavors I like. Each show focuses on one flavor like strawberry or tobacco. I think there are 70+ of them.

Used both over here in the UK and never had a problem

I use perfume apprentice almost exclusively.

It's almost as if OP doesn't know what google is.