98% of crates nowadays are trash like poisons, XP scrolls, and face paints. They had somewhat better drop rates when they were first introduced, but now the rng loot pool of them is diluted so heavily with garbage, it's not worth it.
They measured a 0.75% chance per crate of getting a Lustrous Nix-Ox Fabricant Steed. Averaging the other similar legendary rewards, the chance looks closer to a 0.7%. Over 15 crates, that's about a 10% chance of getting at least one Nix-Ox. Or a 90% chance that you'll have to buy it with gems. I'd expect about 100 gems from 15 crates, so you might have enough.
"I bear the cruel weight of certainty. Total, absolute, relentless certainty. People rarely comprehend the luxury of doubt... the freedom that comes with indecision. I envy you."
―Sotha Sil
Go for it. Use your loan repayments if needed. This will be something you can cherish for the rest of your life. Pass it on down to your children.
Sorry, I wasnt clear on which crates. Go both. That way you can avoid that funny feeling you will have from time to time that you made the wrong choice. Take out new loans if necessary to achieve your dreams.