You will get more readers on the forums early on, and it's not a bad plan to use them as the main base for your promo when starting out. A lot of my most dedicated readers who leave comments and likes are off the forum, so it's definitely worthwhile.
Buuut... the forums do have a sort of cap to them in terms of just how many people are here. Eventually nearly all the regulars will have decided if they want to read your comic/novel or not, and you'll only get subs here from occasional newcomers, so it's a good idea to promote elsewhere too. Early on, it'll feel a bit annoying, because it'll always seem like more effort to get less readers than you'd get from the forums, but it will pay off later.
Remember that a lot of people won't try something til they've seen it a few times, and if they see it in a few different places, they're more likely to try it, so keep promoting on various different social media. 