If someone is willing to give you 10.000$ but you can only use it to buy STEEM or 8.000$ that you can use however you'd like which option would you choose and why?

I'd take the $8000. Steem is just one target that is hit or miss. If some really bad steem news comes out or the price plummets then my investment gets hit hard.  If the $10,000 steem's selling price goes up only 10% that's only a profit of $1000.  Any increase on one investment is uniform no matter how the crypto market itself overall goes.

If I break the $8000 into different crypto investments my investment risk is spread out. Even if two cryptocurrencies don't do so well I should expect a few other investments to cover the difference if cryptocurrency is indeed the future and worth investing in.