Choosing between buying Japanese cards versus English cards is a personal choice; each has its pros and cons. Japanese cards are generally a better quality and their packs and boxes have a higher pull rate than their English counterparts. Japanese cards are also generally cheaper than English, but only if you do your research and purchase smartly. On the other hand, English cards are easier to find and can be purchased at local stores, and you also have the option of playing with them if you choose.
If you do end up deciding to start collecting Japanese cards, I would strongly suggest considering the website
AmiAmi. They are a Japanese retailer that sells Pokémon products as well as many other card games, figures, and other Japanese toys and collectibles. The MSRP of a booster box of Japanese Pokémon cards is US$28 (approximately), and on AmiAmi, they even give you a discount, where you can get a box for around US$22. Compare this to PokéVault that is charging US$55 for a single XY box; that's over twice the price! The biggest downside to AmiAmi is that they don't carry older products, and their ordering system may be different from what you're used to. However, I have used them for a few years now and I highly recommend them.
PokéVault is a nice site, but I certainly wouldn't use them to purchase recent or semi-recent cards because they are so expensive. If you either can't or don't want to buy via AmiAmi, my next suggestion would be one of our seller's here, PokePhD, who sells recent Japanese booster boxes in the Selling section of PokéBeach. Otherwise, there's also Ebay, which is usually more expensive than AmiAmi but cheaper than PokéVault or other retailers. The biggest benefit to buying from places like PokéVault is that they offer older cards and things that are more difficult to locate.