Cannot be told, I think that trading is good on a low price but right now the bitcoin is very high, with altcoins you can to be honest start already now with some nice investing amount.
As I see it, Bitcoin/Fiat trading is a safer option than Bitcoin/Altcoins trading. But, incredible as it seems, the altcoins are showing a good performance in the market, even with Bitcoin rising in price. I think that as long as you have experience you can make a good profit on the altcoins market, but this is a very unpredictable market.
It is right that bitcoin fiat trading is safe but if someone have knowledge about altcoin and start trade with bitcoin/altcoin then he will earn a higher income from that trade and it is also the chance that he get biggest profit within a day.