Well with a modchip, you can play burns, so then you don't need to buy original jp games which will save the most money in the long run. Though if you are planning to play originals only, then a JP ps1 is simply the easiest and best experience. However if you already have a USA ps1 there are many ways to play original JP games without internal mods, depends on what model of PS or PSOne you have.
If you are going to go the import route, spending a bit more for JP PS2 will also allow for imported PS2 games as well, though a few of the popular ones like Gradius Gaiden have issues on a PS2.