that's the first consideration - which alt or token he wants to buy. because you can't find all tokens in coinbase or pancake. so he needs to check first in both platforms or other trading platforms about its availability, and the liquidity as well as fees. he can always compare the approx fees involved because it all boils down to your convenience and fees. he can always check the trading pair if it is avail in the exchange and check the potential amount of fees like trading fee, withdrawal fee. he should start with one alt, because if he will work on several alts he will be confused and tired of doing those tasks.
Actually it's depend on him whenever he buy altcoin on centralized exchange or decentralized exchange like pancakeswap or uniswap.
But from the topic he asked "buying altcoin on metamask" which mean he is using smartcontract coin like ethereum.
So he is not interested in buying altcoins on CEX like coinbase,binance, etc
Using USDT is more stable than BNB, and it's easier to do calculus with a tether. If you buy with BNB and the price goes up then you will get less altcoins, using USDT is more stable at least in one fair.
But usually USDT or BUSD have lower liquidity if we trade on decentralized exchange.