Both are important, proper posture makes it MUCH easier to have proper tongue posture. Most people I find who know about mewing and have already gotten a good feeling of their tongue, experimented pushing up, getting better control, etc., automatically correct their tongue posture when they have proper head posture. However, it's also entirely possible you don't automatically do that, just take a look of the photo below.

You can tell by this photo that the woman is not engaging her tongue, yet she's chin tucking. If you're wondering how it's possible to tell, when you engage the tongue and it pushes upwards, the muscle below the chin engage, the hyoid moves up, tightening up that area where there's usually a double chin. In the past I believed all you needed to have proper tongue posture was to correct head posture, but hearing other people's experience made me realize it's not all that. People told me they couldn't feel their tongue when they chin tucked, which surprised me, but I realized if you're asking someone who doesn't have good control and feeling over their tongue in the first place, who can't swallow correctly, who doesn't really have the "feeling" so to speak that you can't really explain, then it's just not going to work. The only way you get that is through experience, for beginners, it's important to experiment, doing cheesy swallows to feel the tongue get up there, then trying to do it without a cheesy swallow, trying to get different parts of the tongue up, etc. I think if someone does this first then chin tucking will help them so much in getting good tongue posture.

Basically, good head posture is essential to maximize your tongue posture, don't ask me why because I don't know the exact reason but it's just way easier to have good tongue posture when your chin tucking.