Depends. If you prefer farming, buy water. If you prefer mining and have a tendency to fall in lava, buy pyro. If you like mining obsidian, buy water. Don't take my advice though, I don't have either feature
I'd choose water - pyro is extremely useful but you'll likely find yourself placing water more often than placing lava.
I got water. Great for farming and such you get 0 mcmmo xp so you are aware. Then I bought pyro+ + means I don't die in lava when I toggle fire dmg. I fall in to lava about ever mining trip I do so if u like farming water But mining Pyro + is much better
If you're not going to go straight for pyro+ and it's lava immunity, get water because it has more practical uses in terms of minecraft gameplay and can also be used to protect you from lava. I would personally recommend getting pyro+ though it's endlessly useful if you want to do anything in the nether at all.
This pretty much sums it up, Pyro+ is a lot better than water+ but if you can only get the first tier I'd recommend Water
