Why buying hash rate for mining? If you are capable of having a physical farm, why not go for it? Having a mining farm is better. If you have enough amount of money to have a mining farm, it is better and you can get it to the point of having a mining pool. If you do not have enough to set up a farm, just buy one or two miners or have a mining rig and join a mining pool.
why; taxes, technical experience, etc.
edit;
it looks like it is actually the same cost in the end. lol