I don't know that either one is "better," but they each have their perks. I have both, and I use water more, but each of them has their advantages.
Depends on your future plans, really. Do you plan on building a farm and don't mine or go to dangerous locations as much? Get Water. Do you want to get Pyro+ later, which allows using fireworks and lets you not die from lava/fire? Get Pyro. Tell me your plans and I can answer the poll.
Well I plan to make some modifications to my farm later and I rarely go mining and if I do I don't really run into lava.