This is a question which I get asked quite a lot. I have my own personal preference but in reality it is down to you as an aquarist to make your own decision based upon your personal preferences.
I suppose really it depends upon whether the aquarium has water in it or not.
If the [tag-tec]aquarium[/tag-tec] has water in it then you can still drill the aquarium by taking some water out so that you can drill the hole(s). There is a risk here though (as there is when you drill any aquarium with water in or not) that the glass will crack and all the water which is left in the aquarium will come crashing out and you will probably lose a lot of your fish, corals etc as well as having a soggy carpet!
That might not go down well with the other half!
You could, of course remove all of your fish, corals, rock etc to do this but lets be honest you’re probably not going to do this.
In this instance then probably an aquarium overflow box is the best for you.
If the aquarium is empty then I would definitely say that drilling the aquarium is better than using an aquarium overflow box. With the aquarium being empty you will have the option to be able to drill the holes exactly where you want them to be, the size you want as well as the number you want.
You can drill the holes in the base if you so require and configure an [tag-self]aquarium standpipe[/tag-self] type system or you can drill the rear of the tank and utilise overflows.
Another benefit is that if you drill more than one hole and one becomes blocked for whatever reason then the other hole(s) will still operate and the aquarium will not flood and the sump will not run dry.
The aquarium overflow box work normally on a siphon based system. This does have its disadvantages. The main one is that the siphon can stop. When it does the water will stop overflowing down to the sump yet the sump return pump will still be pumping water back to the display aquarium. Therefore the display aquarium may overflow and the sump will run dry probably damaging the pump.
An option to overcome this is to install more than one overflow to cope with this problem, however aquarium overflow boxes can be quite expensive therefore whilst beneficial it could end being costly in financial terms.
With both drilling an aquarium and installing an aquarium overflow box it is important to size them for the amount of water flow required. This is based upon the flow rate of the return pump(s). I would also recommend that you add extra flow to the requirements to cater for the possibility of blockage. Also remember to consider the head loss of the pump(s) when doing your calculations.
I think that you can probably see where my personal preference is – drilling. Of course an aquarium overflow box does have its advantages and if you are unable to drill the aquarium then they should not be discarded as they are a fairly easy option to install.