It's an oft-repeated mantra around the Internet that there are two kinds of writers -- Pantsers and Plotters. The definitions of these two styles changes depending on whom you speak to and what time of the day you talk to them, but the standard goes something like this:
Pantsers are the writing equivalent of "shooting from the hip." A Pantsers sits down at the computer and writes the first draft without any sort of planning or forethought on where the story is going to end up. They have a character, or a scene, or a series of scenes and write to those without consideration for how things connect in the greater whole of the story.
Plotters, on the other hand, spend copious amounts of time in the planning stages before they write a single word down. They have piles upon piles of notes on character interviews, scene sketches, outlines, etc to the point where they already have a rough idea of the opening, rising action, climax, falling action, and denouement of the entire story before they've even typed/written Chapter One in their novel or the opening words in their short story.
There's also the Pantser/Plotter blend, where the writer will "shoot from the hip" for some sections, but still has a general outline of where they intend the story to go. These people might have pages of notes that they've taken down and don't use, or may have started off with a sketched out idea but not kept to it.
Now we come to the question I posed in the post tile: Is it better to be a Pantser or a Plotter?
For my money, the answer is that the type that's better is the type that works for you. Case in point: Cynthia Reese is an avowed Plotter -- she's said herself that she plans practically everything out before she writes a word on her novels, whereas Tawna Fenske is a Pantser through and through.
Two writers, both published with contracts, and they have two different writing styles. This should go further than anything I have to say to show that your writing process doesn't matter so much as writing a good story does. Whether you fly by the seat of your pants or plan things out in minute detail, the only thing agents, editors, and eventually readers care about is that your story is interesting enough for them to read.