Hardwood is more expensive, more difficult to work with and handles moisture worse than plywood. I build my first fish room racks from hardwood and despite sealing them well, they would warp - twist and bow especially if weight changed.
If your set on making a hardwood tank, the best advice I can provide is to make a much smaller version to test out your materials and techniques on. Your the only person who will know if the extra effort required is worth the end result.
Most of the experienced tankmakers have left this forum unfortunately.