Processing is the most popular among artists, and so it has the most artistic tools ready to just drop in and use. If you want to focus on the art-making, it's your best choice until you feel you've outgrown it. You can just jump straight into brushing and blitting and blending and whatever without having to scour GitHub or craft something yourself from bare metal.

The only caveat is that if you're coming from a programming background (and it sounds like you are), it feels a little rudimentary and wonky. You might sometimes feel your style of approaching a problem straining against Processing's control flow and language constraints.