The left side looks great, but the right side is too rigid and unnatural.
If you can, for a more natural look, use wood that is thinner. This will help give the tank a better scale. Using smaller items makes a tank look bigger. Right now I cannot tell it is a 3 foot tank, to me it looks at most 2 feet.
For example, this tank is only 12 gallons, but the thin wood makes it look much, much bigger.
The second set up is def better, but maybe a bit too symmetrical. Personally I would leave the wood on the left alone, and move the wood on the right to cover more of the side tank wall instead of the back corner. Nothing really wrong with it though, it looks nice!