Base Set and 1st Editions have shown consistent 10-20% annual growth
The fastest way to check is by looking at the cards texture, holo finish, font spacing, and card back colors
If you want one for your collection as an historical trinket, I recommend reaching out to Keith Adams on facebook or similar and he can probably hook you up
The other types are self-explanatory
Which form do you like best
If you've read my reviews of those cards you'll know that I thought both Froakie and Frogadier were extremely poor, since neither could do damage every turn, however, if you base your water type deck around Greninja BREAK, and have multiple Greninja in your deck, Frogadier's one move will help you get Greninja into play much faster since it will allow you to have multiple Frogadier on your bench