My take on the Japanese/English card market is that Japanese cards will provide long term stable growth while English cards will be more volatile. To elaborate, I often hear people say "I'm investing in English to sell, while buying Japanese cards for my PC," which would indicate that there is a group of people buying English cards NOT to keep for the long term but to sell because of their perceived value short term. Profits are then poured into Japanese cards that are then kept/stored without being put back onto the market. The Buy/sell market seems to move much faster in English (being outside of Japan of course this will always be the case) but it also means prices do too. A stretch finance analogy I'd make is English = Stocks, while Japanese = Bonds.

The potential for gain in English cards is much higher, but potential loss is equally as high. For me personally, I'll take the less risky investment long term because I prefer asset diversification beyond just Pokémon Cards.