The S&P 500 has beaten the tar out of metals, so by "long term investment" one must mean they mature during the fun cockroach wars when civilization ends.
Except the stock market is also just another imaginary market, that is now powered by a routing and checking number. The stock market means and holds absolutely zero value, until you have that value in hand. Only until its in your hand, is it actually worth anything.
Seeing as most people cannot withdraw their 401k, its an imaginary "savings" account for 30+ years. Which could be worthwhile or worthless. No one can forecast that far. Me, being 37 years old, and slowly accumulating gold over the last 10 years, whilst still investing at least 15% of my savings into my 401k, I would like to believe gold will always hold some sort of value - whereas stocks and indexes get shuffled around every few years.
Gold and silver will be at least worth something in 30 years, unless they discover a new metal to take the place of gold, silver, platinum, iridium, etc. "precious metals".
I'm 37, two paid off vehicles, and 1.5 houses paid in full. I much prefer tangible, hard physical assets (real estate) -- that really excite me. I don't get giddy anymore about dreaming of "having" $2,000,000 in a "401k" which may or may not be viable in 20 more years. My primary residence and primary vehicles are paid off before 40 years old - I'm LIVING the American dream. And I cannot be more thankful or grateful. If I pass away tomorrow: I'm totally happy with my life decisions. I live zero regrets and will never be a financial burden to my family of 5.
Be much harder to take away property and physical assets.