You're doing what a lot of people do, especially in the situation you're in...looking at things in black & white. There's a HUGE grey shade here.
It's never an issue of molecular level planning versus ready-fire-aim. Those are the extremes. Find your answer in the middle.
You don't say how big or complicated your project is, so let's assume there's a certain amount of complexity or you wouldn't be posing the question. (On the other hand, the fact that you're considering two radically different approaches makes me wonder just what kind of a project you've been handed.) My advice would be as follows:
1. Remember the seven "P" rule: Proper Prior Preparation Prevents Piss Poor Performance. Which is to say all things require some planning, or in the words of that famous philosopher Yogi Berra if you don't know where you're going you won't know when you get there. Even the smallest project needs to be planned but you need to determine the appropriate amount of planning.
2. Don't sacrifice the urgent for the important. Don't make the mistake of buying into the idea that "something" can be done quickly just because Guy At The Top thinks the project can be knocked off while you're having you're morning coffee. If in your professional opinion you need days/weeks/months/whatever, then you need to document and make your case. Use business and financial terms to do it...speak the language of your customer, or what GATT will hear will be ancient Greek and you'll be stuck doing what you know can't be done.
3. On the other hand, be tactical first and strategic second. GATT has a business to run, and you have to do whatever is necessary to support that. Don't worry about being perfect, and don't worry about installing spaghetti (code or wires). Get to a deliverable quickly, even if it's something small and barely useful. It'll establish your credibility, give the users something to work with, buy you some time to get more work done, and provide a tactical platform from which you can springboard into a more strategic solution.
