Either way can work Matty,
I went with the 2by4 frame on the outside for extra strength and support.
My tank is in the livingroom and i have two kids and a large extended family, so I had to make sure that it could never tip over no matter how much pushing or rocking it took( its 1.2 ton).
Also because its 3ft deep I wanted to be able to climb on it to do maintenance, you can't just swing your arm in to clean the glass when the top of the tank is 6ft high and the waters 3ft deep lol.
I've seen tanks this size with no bracing that have worked fine and have been told a few times that I had totally overbuilt the stand and supports. Without bracing you would need to be anal about anyone leaning on or touching the tank and who can be bothered with that.
The first thing a mate did when he first seen the tank was try to rock it back and forth to 'test' it
i didnt build this as in experiment on the limits of water pressure vs plywood and screws and always thought the overbuild arguement was nonsense. It's usually the same people saying you've overbuilt that come back and say how the tank has a limited shelf life of a few years when compared to all glass tanks. When I was researching my build I found guys with ply tanks 20+ yrs old because they were properly supported.
As long as I could go to bed knowing that I wouldn't come down to a flooded livingroom the extra £30 for the 2by4 tank frame was a no brainier.
The supports outside mean that the water pressure is evenly distributed along the plywood and stops any hairline cracks in your fibreglass caused by the ply flexing.
As you can probably gather by now my advice would be to go with the outside framing, trust me when the tanks built you'll be glad you did and can enjoy your tank without that 'what if it pops some day' fear in the back of your mind.
Sorry for the long post, I'm rambling a bit as I'm tired ( insomnia's a b**tch
.