One is set up to lose value over time (the US$ has lost 99% of its value in the last 100 years) and the other is set up to rise in value over time as supply is managed to a fixed endpoint
I call BS due to the fact in most countries you get a real return of around 1% a year in fiat by keeping your money in the bank. Even more if reasonably invested.
Furthermore, for people stating that there is an unlimited supply of dollars, that's BS too. Central banks can forcibly take Money out of the system when inflation happens by increasing the cash rate, selling back government bonds, leading to a reduced money supply. This all happens transparently. BS on the closed doors. Good luck holding inflationary bitcoins when the united states starts tapering. Bitcoins are to be continuously produced for over 100 years still to come , meanwhile dollars are going to be constantly destroyed in the near term due to the taper. Then money supply will converge to mean values dictated by average interest rates. Those averages are much higher than rates today. Thus cash will become more valuable in the long term compared to now, provided it is hedged for inflation either through term deposits or inflation linked bonds.