The main question is in the title.
I am not too familiar with BitShares, but as far as I can see, their platform only allows you to trade cryptocurrencies. In Bisq you can trade fiat with Bitcoin on a decentralized P2P network.
Bisq and Bitshares are my two favorite DEXs. With Bisq it is possible to exchange FIAT for BTC, the transfer is made from Wallet to Wallet.
Bitshares Dex allows to change the IOUs of the cryptocurrencies provided by several bridges. But IOUS requires a third party, on the other hand Bisq does not need third parties. However, the rates of the Bitshares tx network are the lowest and fastest of all the blockchain.
Another interesting aspect of Bitshares are its Smartcoins. Self-guaranteed tokens, without bridges or third parties. They maintain their value through margin transactions. For example, 1 BitBTC is worth, at least 1 BTC, guaranteed price without third parties.
It could be interesting to be able to trade in Bisq with BitBTC, in this way the network rates would be very small and the speed instantaneous, which would allow an agile operation. At any given time, BitBTC could be exchanged for BTC when it decided to conclude operations.
I know that they use special bots in telegram for p2p deposit-withdrawal operations using bitCNY, bitUSD, bitRuble and others pegged smartcoins for that.