I think buying is a safer, simpler and lazier option, which can also bring some long-term profit and easily make $40k into $100k over a couple of years. As for mining, it means not only initial investment which you that need to cover by profits (when you buy Bitcoin, you can always pull your money out, and you'll get more out of it than out of selling mining equipment, probably). You also need to cover the electricity bills, which can be huge. Not to mention maintenance, so that your equipment doesn't break. And, possibly, taxes for doing business, maybe some permits to do that etc. So I'd go with buying Bitcoin.
Lazier is the key word here. I mean don't get me wrong, if you have the money for it, the tech mind to keep it going, and have the mechanic talent to fix anything that may go wrong then go ahead and do that if you want to. But for the life of me, I do not get people who have none of that and still end up buying bitcoin or gpu mining systems.
One wrong move and your investment could be gone, one broken gpu and your money is gone. Just buy it, hold it, and literally do nothing about it and you could make a good return from it for sure. I personally believe that we could make a good return from it without doing any work at all and that is a great deal.