why should we choose between these two only? and why fiat? you don't invest in fiat, investing in it makes no sense. you can invest in a lot of things like gold or real estate as the alternative to bitcoin.
as for crypto. it is silly to invest in altcoins, the only viable investment is bitcoin. altcoins are too risky and manipulated to be invested in.
It's possible if you really want to debate on that matter (off topic).
You can invest in fiat if your goal is to put that fiat money against your own. As an example we can use someone who uses Turkish Lira and owns a bunch of it. That currency's inflation is high, so many other currencies will go up against it. If you decide to buy dollar that appreciates greatly against your currency and sell it after a few months, you'll have a lot more lira. Doing that makes you bet on the lira's ability to eventually curb inflation and stabilizing, so it can be an investment.