I definitely err toward the heavy planner option. Like, I'm far from completely rigid- dialogue especially frequently changes as I get to the page it's mean to go on, many side/minor/background characters I design on the spot when they have their first appearance (for better or worse lol), the events within a pre-planned scene might adjust somewhat based on nuances I tweaked in previous ones, and my paneling from my story board is always being changed and rearranged.
BUT I need my gameplan in place to have the confidence and drive to push a project through to completion lol. I've tried many times in the past to jump into a comic with little or no upfront planning and was never able to get very far with them. Having the whole thing plotted out gives me a tangible path to follow and milestones to hit, which is super motivating~
Granted I've so far only been working on smaller scale projects: I have a 10 page comic under my belt and nearing the end of a +/- 70 pager. If I were doing something longer- like several chapters long- I have the feeling that I would plot the whole thing out first, but only go into detailed scripting and storyboarding on a chapter-by-chapter basis, thus allowing some of those "allow the story to grow as you go along" aspects. I'm actually in the process of writing a project like this but it'll be a very long time before I'm ready to begin it, I feel. Much more planning to do first LOL