I'm learning Chinese from a book, and there are some examples regarding the use of `还是`. In some cases, the verb is repeated, like in `你喝茶还是喝咖啡？` (note the two `喝` here). In other places, the second instance of the verb is omitted, like in `他学习日语还是英语？`

Is this a question of preference, or there are some rules to omit the second verb?

Thanks!