In my opinion, muscle should be functional, especially if you are hoping to build and maintain a physique that is ideally suited to your genetic endowment. Find a sport that you enjoy and train for it. You'll gain a well-honed physique, good general health, and the immense enjoyment that accrues from being in the moment, making the most of your time.
Muscle for its own sake shouldn't become a fashion statement, and if sought through a superficial approach, it can become difficult to maintain the motivation to do the training. Find a form like swimming, tennis or martial arts that gives you a reward in itself, and you'll gain a vitality and lust for any fitness training designed to augment it.
In my experience, learning karate was inspiring, and I saw beneficial gains in muscle definition through rigorous workouts in the dojo, supplemented with resistance training in a gym. I didn't became an elite athlete by any stretch, but I developed substantially in my late 20s and early 30s. And I had a lot of fun and satisfaction doing it.