I would recommend you go with a rectangle. Cubes are great if you have a very specific plan/goal in mind or are limited in space. The only real benefit cubes have are that they generally speaking require less lighting and potentially power heads for flow. However, they have a long list of (potential) downsides.
1. Your sump size will be restricted considerably, and you'll be required to either a) buy a cube-specific sump which is usually more expensive than a DIY-possible glass tank with baffles. If you go with a "cube" sump you'll be fighting with yourself whenever you're shopping for a skimmer, return pump, etc etc. b) Get a regular sump and keep it outside of your stand, but this could potentially be unsightly or c) build a stand that is much longer than the tank in order to accomodate the filitration you need. This can work, but reinforcing the top of the stand for the footprint of the tank can be more convoluted than your typical DIY construction.
2. Cubes are shorter in general which means it will be more difficult to keep fish that grow past a few inches, and anything that generally enjoys swimming will feel restricted in the space.
3. This isn't really a con, but I personally think aquascaping a cube well is considerably more difficult than your average rectangle tank. Cubes require important focus on managing both depth and height due to the restricted length of the tank. Most people enjoy their tanks for the aesthetic, so this should not be overlooked.
