The big difference being generally one or more governments are willing to back a currency. Who will back bitcoin? It's not a currency just because some people will buy it hoping to sell high.
The thing about bitcoin is that for all the talk, it's not really being used as a currency. A currency is for purchasing goods and services (and paying taxes). Ok so theoretically you can buy things with it from an incredibly small selection of vendors. (You would have had a bit more, but not much, opportunity in the recreational pharmaceuticals market until recently, but 99.9999999999% of that still uses traditional money). Mostly, bitcoin is about get rich quick. Either through turning electricity into money through "mining" (pretty much unviable now) or through buying low and selling high. It really is just a big Ponzi scheme.