Almost the same thing... the ETF with the ticker SPY.
When I researched this years ago, I concluded the costs were less expensive than any S&P 500 mutual find I could find. Not sure if that's still true or not.
They are also popular because the company is owned by the share holders of their funds. So huge dividends and salaries aren't going to employees or shareholders.
But of course fees are effectively much higher if you are paying transaction fees.
But vfiax (vanguard) or swppx (schwabb) can't be beat (.04 and .03) when it comes to fees. And it is pretty common for brokers to offer free Mutual fund trades or you can get an account directly with those brokers and trade for free.
Don't forget iShares too, which are usually traded commission free for many brokers (I know for Fidelity and USAA...). IVV has an expense ratio of 0.04% and they have other really low cost ETFs too.
I think there are some additional internal costs with ETFs that cause it to have lower returns over time. At least, I remember seeing some comparison graphs to that effect.
SPXL if you want to play with leverage. Wait to see if the commander tweets 5 minutes before markets open before buying in on any given day, it will help.
Many companies offer index funds, though Vanguard's seem particularly popular (possibly because they invented index funds, IIRC).