I mean...it's not a fine line, those are two completely different things. ^^;
Personally, I prefer the former, although I agree that you should practice with shorter projects before starting a big one.
FINISHING is a really big deal when it comes to storytelling in any form; most amateurs are only used to starting and propagating. Setting an end goal that's actually in sight and actively working toward it is a different ballgame, and it forces you to take your work seriously in a way that you often don't learn to do otherwise.
Here's the thing, though: you're gonna learn while you work on your Magnum Opus. It's inevitable (I've only been working on mine for 8 months, and I've already learned a lot). Whether you get better with art fundamentally or you just learn how to do what you do faster and easier, you're not gonna be the same artist at the end that you were at the start.
That's why you shouldn't get hung up on waiting until you're 'good enough' to start a big project. You've finished a comic before? You got your art style about where you want it? You're ready to start. It's gonna be a long journey; don't forget to have fun~