One stablecoin, several networks: ERC20, TRC20, BEP20, Solana, Polygon and others. Fees, confirmation speed and merchant, wallet and exchange support differ between them.
When paying an online merchant, what decides your choice first: the network, the fee, or where your funds already sit?
Every factor should be considered especially the fee. ERC20 is usually expensive from time being, people avoid it because of high transaction fee, except if it is compulsory to use the network, one can avoid it for any other available network.
Secondly, you will look from the wallet you are sending to it will determine which network to be used, but it also depend on your exchange, if the network withdrawal is not available, you have to chose the available network and not the cheap one.
Almost all the networks are fast depending on your internet connection, make you choice using any factors.