There is an eternal and never-ending battle between numismatists and philatelists. Who is the "hobby of kings" and which one is "king of hobbies"?
Answer: A hundred years ago and older only wealthy and very rich could pursuit collect coins banknotes and postage stamps. That's why they called the "hobby of kings".
Only rich had ability travel between countries and Kingdoms. Keep in mind that coins were minted from precious metals like gold and silver. Even in these days collection of valuable coins requires a lot of money. Now times has been changed. Coins are made from cheaper alloys and the banknotes are usually expensive than regular coins.
Philately (Postage stamps) was also a hobby of wealthy people who had connections around the world. Ordinary people could not collect many different postage stamps, no doubt it was not valuable.
The hobby of collecting coins and stamps are both equivalent. Real kings had ability to collect both at the same time and not limited to that.