So external over flow is ok. Can't decide on it. Can you still do plumbing with external overflow ?
As a general rule, internal overflows are the standard, and are commonly available, with minimal chance of failure.
When you say external overflow, I assume you mean a hang on back style overflow box. Some people will tell you that they've used these for years with no issues, and others will tell you horror stories. These don't take up tank space, but have potential for disaster.
The best solution by far in my opinion, is an external scupper box. This is basically a built in overflow box, but instead of a huge black piece of plastic taking up tank space, it is siliconed to the outside of the tank, and doesn't require a siphon like the external overflow boxes. The downside is that they are uncommon, and can add to the expense of your tank significantly.
My suggestion:
If this is your first tank, get an internal overflow. An external overflow is prone to issues, and is one more thing to worry about. A scupper box is the way to go on your last tank. Once you've made all of your mistakes (you will make mistakes), and determined that your next upgrade (you will upgrade... yes, you will... no, I mean it, trust me) is your last, then go with the starphire glass and the external scupper box. Unless of course, money isn't an issue, in which case, go with the best from the start.
