I wouldn't drill unless you have experience and it is a brand new tank. Older tanks crack a lot easier, and even new tanks can crack pretty easily (especially if you are not used to drilling glass). I did it at the shop I was running and the thicker the glass the better. Thin glass cracked a lot easier.
I have an overflow box and had drilled tanks at the store. If I did it again I would probably still go with an overflow box. I think it looks great, is easier, is arguably safer (a bulkhead could leak and drain the tank down to that level).