While both are done in both decentralized and centralized exchanges the latter have a more lighter trading model.
Crypto trading can be on decentralized exchanges, while most decentralized exchanges are not fully decentralized. But forex brokers are not decentralized in design, they are completely centralized.
The major difference between the two is that in cryptocurrency trading there is no middle man but in forest trading the middle man are the life wire of the market
Making use of centralized exchanges in crypto trading , the exchanges are the middle man. Making use of decentralized exchanges in crypto trading, the decentralized exchanges act like the middle man but in a decentralized way. Only crypto trading that requires no middle man are p2p if done directly between two people not on exchanges. Forex can also be like that, in a way there will be direct trading of fiat between two people.
Secondly in forex there is highest liquidity volume but in cryptocurrency trading there is low or no liquidity once you deviate from the major coins e.g bitcoin and ethereum with a few other coins.
That applies to shitcoins, in this regard, the first 80 cryptocurrencies worth each is worth over $1billion while over 100 of cryptocurrencies each is worth over $500 million. Making use of good exchanges with high marketcap for fast liquidity is better in this regard and do not deal with shit coins. Although, the fact still remains, marketcap of forex is higher than that if crypto.
forex trading has more security and regulation but with minimal profits/rewards
Although, I prefer crypto trading, but it depends.