The answer is fees. Formally, they call this the expense ratio. This is the amount that the ETF shaves off the top every year to pay for managing these funds.

However -- unlike gold, storing bitcoin offline costs nothing really. Keys are relatively small and sufficient entropy from 24 words will last a long time.

The professional custody services used by the ETFs -- Coinbase Custody and Gemini Custody, do however, charge fees. This is in return charged to consumers + management fees of the ETF.

One of the biggest problems with GBTC was the expense ratio of 1.5% -- 1.5% of your coin went to Barry. With Larry only charging 0.15%, you can see why people told Barry to sell their coins so they could go give them to Larry.

Over a long time scale, the fees will eat into your principal. Bitcoin has no dividend, so your principal (denominated in sats) will decrease, but you wont notice because the market reports the price denominated in $usd.

You are best to go with the ETF that has the lowest fees, if this is where you wish to store capital. Look for the one with the lowest fees.

Today this is BITB with the lowest expense ratio.