Question:
What hardware should I get? Is it still worth going for the Antminer S9 for BTC or is it simply easier and more profitable in terms of ROI + long-term if I go for Altcoins?
Even though Antminer S9 is efficient, it's not that durable. It's just a matter of time before it doesn't work (to an extent) and that goes for other Bitmain products (not that reliable), so in the end, the ROI it becomes more than you initially calculated (even without factoring the difficulty changes).
In regards to altcoins mining, there's been a significant decrease in profits (in recent months).
If you decide to go with
BTCitcoin mining, go for Avalon's A7 (less efficient but very durable) and if you decide to go for altcoins mining, it looks like a gambling at the moment, but the re-sale value is still great in most cases (for high end GPU's).