I'd go for the fire... as said above, if you're strong enough, your flames might be hotter as well.. and since fire can be a couple of million degrees Celcius, and water 'only' -273 It's easy to think of the outcome

Even if you can't strengthen your powers(the anime never said you can,right?) , fire just looks cooler than anything else
Against other devils fruit users, water is definately superior, since all you have to do is hit them. But in the fight between Ace and Smoker, I believe that Ace said that fighting would be meaningless, since they can't hit eachother.