Both. All forms of learning are good. Play games in real life, on the internet with slower time controls, read books, solve puzzles. All will further your development.
This is assuming you're not at the stage where you randomly drop a piece here and there, though.
Having not played for over 20 years, without giving my age away I'm a wee bit rusty. What do you think is the best way to pick up the fundamentals of the game, find a club or play a few games on the internet where I have more time to consider the right move.