Honestly, I have no idea.
On the one hand, I used the N-Zap '89 during the last Splatfest and almost always won with it.
On the other hand, I haven't used it since, and I might use the Octobrush or Inkbrush for this one. Or maybe one of the Dynamo Rollers, I've been trying those out lately.
Ooh, but on my imaginary third hand, I've got to wait until I see the August Update's weapons before I make a definite decision. Between the Splatling and the Bucket Slosher and whatever else we get then, I dunno what I might end up picking up for Splatfest.
But if I had to choose right now, I'd probably use the Octobrush. I'm really feeling it right now, and it's pretty good in general, at least I think so.