Boils down to practicality and situation. As mentioned "finished" junipers generally go in unglazed stoneware. If it's show-bound, then a very nice stoneware pot.
You can grow junipers in most any container, from plastic to stone. However, plastic can deteriorate--depending on what kind of plastic it is--with sun exposure and time. It can also "flex" when picked up and moved around, which isn't the best thing for developing root systems. Ceramic grow pots or actual bonsai pots don't do that.
It's up to you really, depending on costs involved --ceramic pots cost more, but they also last longer (sometimes longer than the trees in them). Plastic pots are cheap and available, but they're ugly for mostly developed bonsai.