Traditionally unglazed... but if there are no pottery police around and you like a glazed one, it's up to you, as long as the inside is unglazed.

Two strange things to note. There are no such things as pottery police ;-) The choice of pot is up to you and the tree.

Unglazed pots work best with conifers because they are very understated and generally the image presented with conifers, especially junipers, is austere. Glazed pots can, and do, work with juniper, just as unglazed pots work with some deciduous trees. Paring a pot with a tree is very much a taste thing. It's the same thing as trying to much match a tie with a suit and shirt. Many things can work, but some pots are akin to the polka dot tie your mother in law gave you one Christmas--they simply won't work.

As for the interior being unglazed, that's not really true either, but you won't find a quality bonsai pot with interior glazing beyond the top 14" of the inside rim. Interior glazing can make trees unstable in the container, as the roots have nothing to grip onto. Unglazed quality bonsai pots have a rough interior that allows roots to adhere to the sides more easily. That lets the plant stabilize itself. They slide around in a glazed interior pot...