Some things to help you with you're decision, what species would you like to live in the vivarium? Some, such as imitators will use as much vertical space as you can give them, whereas some other species will stick pretty much to the ground. Either way I really prefer to go at least 18" tall, it really opens up possibilties for planting...if you've ever planted a ten gallon or smaller tank you should know what I mean.
Another thing to think about would be access to the terrarium. Once you get up around 24" tall, a top opening tank becomes increasingly difficult to work in, which is where a front or side opening really comes in handy. If you plan on breeding in the tank, you will need to get in there more often, for example, my auratus lay a clutch of eggs every week, in the ground litter, if the tank was 20 or more inches tall it would be kind of tricky to get the eggs out.