There's also an important distinction between Japanese releases and Asia releases (Hong Kong). From what I've read, the games themselves should be interchangeable and compatible on either marketplace when it comes to DLC /when the Asia release is in full Japanese/, but if the Asia release is in English/Chinese/etc, it won't be compatible (only with the Hong Kong PSN in that case, I imagine). Asia releases are typically quite a bit cheaper as well ($60 vs $80 from what I've seen). As long as they're both strict Japanese releases, the only differences should be on the box-art and possibly the inserts (the Asia releases have no CERO rating, or any rating whatsoever).
An important distinction for me personally as I was planning on sticking with Asia releases for the cheaper costs of import, and I was unsure if I was better off making a Hong Kong account (for more guaranteed support with any DLC I get with a game or decide to buy, as well as cheaper prices for DLC on both stores), or a Japanese account (which would likely have better availability on the marketplace, Project Diva for instance I've heard is interchangeable but the HK market is missing DLC releases). Think I'm going to go with a Japanese account just to have the larger selection overall, and in regards to Asian imports, I just need to make sure they're full-Japanese before buying to know it'll (most likely) be compatible.