Main comic right away.
Making comics is incredibly time consuming, and life is too short to spend my life on projects I don't care about.
Edit: (sorry, I can't get the spoiler to work for some reason
) Should go without saying, but this is true for me, of course. I had ONE story I really wanted to tell, which I'd already partially written, and a whole bunch of shorter story ideas I either didn't care enough about or that were nowhere near as developed yet and would have required even more work + additional skills I don't even have. Scrapping my "big" project to work on stuff that had to be done from scratch and/or that I didn't care enough about just because it was "shorter" would have taken me a lot more time/energy than just jumping right into the thing I really wanted to make. Am I going to make mistakes? Of course I am! But at least I'm going to learn while doing something I truly enjoy
As I said many other times here on the forum, the only thing I managed to achieve by "waiting for the day my skills would be good enough to make the perfect comic" was wasting five years of my life while not actually improving on anything. Obviously things are different for everyone, but jumping straight into the one thing I really wanted to do was what did the trick for me.
Not to mention that, as the time passes, certain stories/genres tend to become a little... well... outdated: think about science fiction, for example. A lot of the "classic" works nowadays still stand because of their message, but stuff like technology is completely outdated and makes no sense at all. Flying cars for everyone in 2020? Hahaha... no :'D
Granted, you could always rewrite the pieces of your comic that don't feel up to date anymore, but that's A LOT of work that could have easily been avoided if you started it when you wanted to. It's easier to forgive a sci-fi story in which the technology gets older with time, but to start a story with outdated technology right away? ...Eh.
Alsooooo: nothing grants you that once you're finally done working on the whole thing, somebody won't have come out with a similar idea that got much more popular than yours, so that now it looks like -you- copied the popular work. Being in the right place at the right time is extremely important to success.
...Which brings me to the reason why I jumped into this topic. Truth to be told, I don't really agree with the whole notion that "if your comic doesn't get a steady growth, you better leave it aside and focus on something else". I see this A LOT on these forums and... honestly, I think it's not a very constructive way to look at it. Many, many, MANY famous artists whose works we still love nowadays went through A LOT of failures. Disney, Tim Burton, Van Gogh... heck, Paul Cézanne didn't sell a single painting until he was 60 years old and he didn't become "famous" until after he was dead. The reason why they became famous was not "giving up after the first failure"... but continuing despite their failures. The Internet nowadays makes it so easy to measure your success with numbers, but thing is... those numbers are really not as objective as one may think.
Take my own comic, for example. When I started to post it (in a slightly different format), I had over 5000 followers on my Tumblr blog. The first chapter got 85 likes (around 45-50 just on the first day), and although the following episodes didn't get as many likes, I still had quite a bunch of people following my story, commenting on it, heck... someone even drew fanarts and once I stumbled upon a blog post that praised the story. Now, compare to Tapas. The comic has 46 followers, one of which is yours truly. When you put the likes of every single page together, the first chapter has less than 50 likes. I didn't get a single new follower for the past month, despite the fact that my comic is always in the "popular" category on Tapas. If I were to base my success on my Tapas stats and act accordingly, I would have already given up. Clearly the comic isn't "flying", so it must mean that I'm doing something "wrong", right? ...And yet there was the Webtoon version, which got over 40 followers during the Canvas Marathon week. Same story, same artwork. Is the story "better" on Webtoons? Heck no, it simply got more visibility on there, which helped me raise my number of followers.
In short: I know that if I could get people to like my comic in its old format, I can still get people to like it now. And I know that, if given more visibility, my story has the potential to grow a lot more than it's currently doing. So no, I'm not giving up on my "big" project just because of a bunch of numbers on a single online platform that gives me very little visibility to begin with.
I see plenty of great artists here on Tapas with amazing drawing and storytelling skills with less than 50 followers. Some are just not very good at promoting themselves, some others try everything they can, but write a genre that is simply not popular on this platform... honestly, there's A LOT of reasons why something may not fly right away, but this doesn't necessarily mean that your project is "bad", that there's "something wrong" with it or that you should "give up to make something that will be more popular". Granted, if you're looking to make a living out of your comic, that may be the way for you. But for everyone else... I'd say just do whatever you want and stop chasing numbers. Focus on finding marketing strategies, promoting yourself and for the love of God, don't focus on a single platform. Being talented or writing a story that fits into a "popular" category isn't enough to make you successful: marketing, being in the right place at the right time, hard work, not giving up and a little bit of luck are important too.