under fidelity FBTC the OP does not own a share of BTC
he owns a share of a fidelity company share. he cannot redeem that share for actual BTC. he has no ownership rights of actual BTC
however yes using the 401k method to own shares EXPOSED to bitcoin PRICE. means he gets the exact same ups-down of the market movements, with for him the upside that he can purchase exposure amounts at a discount due to pre-tax share purchasing. and tax free selling of shares
but its still worth stating the OP does not have ownership rights over actual bitcoin.
With an ETF shareholders, admittedly only the larger ones, have a right to redemption meaning they can request delivery of the underlying asset. To simplify the Bitcoin ETF's it is a cash only redemption meaning you would get the spot market price of Bitcoin not physical Bitcoin but effectively it's the same thing. So with an ETF in a very real sense the shareholders are owners of the underlying asset and the fund manger, in this case Fidelity, is just a custodian. It's not as good as holding Bitcoin in your own wallet but has the advantage that you can get access to it through an IRA or 401k with a linked brokerage account.
i can tell you have not even read fidelity's s-1/a filing with the SEC..
there are too many corrections i would have to make to clarify where you have things mixed up, that its probably best you go read the s-1/a to realise that you as a share holder through a broker are several levels away from having any ownership claim of bitcoin. your shares are just measured on the performance of the bitcoin market index..