If you are interested in arithmetic it is more profitable to buy bitcoin. But there is a significant problem. You for life, you must have the Fiat. You will have to sell some coins to provide for their needs. In this case, the number of your bitcoins will be permanently reduced. Coin mining will always be profitable.
I have this in excess of what i need. Therefore the $1500 is here, and ready, to invest.
My question which investment is better?
I think in these times where time is critical you are better buying bitcoin than mining, mining could give to you better profits in the long run, but taking into account all the time you need to wait, the taxes you need to pay and the added cost of mining, it will take you a while for that to happen, in the other hand buying bitcoin right now is something you can do immediately and if bitcoin skyrockets in the next year your profits will be better than if you bought another miner.