Even ignoring the studies people have referred to in this thread, we can expect that someone leaner will have an easier time building muscle for a few reasons. As your body fat percentage increases, so does your insulin resistance. As your body fat levels increase, so does aromatase, and the conversion of your testosterone to estrogen. Overall being leaner is going to have a positive impact on nutrient partitioning, up to a point anyway, there is such a thing as too lean, where your hormones can suffer.

So in a caloric surplus, the leaner person should build more muscle. In a caloric deficit though, the tables are turned. The leaner you are, the more likely your body is to favour giving up muscle instead of fat when it's deciding what to use for fuel. If you're 10% body fat, and cutting down, you're not particularly likely to gain any muscle, even in a slow cut, you're mostly just trying to preserve the muscle you have - but if you're 25% body fat, and you're cutting, as long as the deficit is kept moderate you should be able to gain a reasonable amount of muscle even while maintaining a deficit.

That's why I'm not a fan of the "it doesn't matter how high your body fat % is, if you're skinny you have to bulk, you have nothing to cut down to" advice I see a lot - because that's simply not true. You can absolutely gain muscle in a deficit, and the higher your body fat % is, and the less muscle mass you have, the easier that becomes. So it doesn't matter that you have "nothing to cut down to", as you'll be gaining lean mass despite your scale weight dropping.

18% isn't exactly fat though, it's kinda in that middle ground whether you could go either way. Just be aware that the higher your body fat % when you start a bulk, the longer your subsequent cut will be. Some people are good at really long cuts - me? I can't do them, I get impatient after a while and start going over the top with my deficits, which is a good way to lose those gains you've worked so hard for. I'd rather get lean first, then bulk to a point where I only have an 8-12 week cut ahead of me (or less), than bulking to a place where I'm left cutting for 5-6+ months.

Generally it comes down to you. What bothers you more about how you look right now? Your body fat %? Or how much muscle mass you have? Address the one that bothers you most, and be mindful of what that choice means: i.e. a longer cut if you decide to bulk, and potentially looking very skinny if you're a beginner and decide to cut first. Either way though, the trade offs are temporary and whatever decision you make will bring you a step closer to your goals. :)