You can use either, and so do most people. BTC's price fluctuates a lot just like the price of pretty much all coins(except USDT), so expect your trade profits to be fluctuating a lot too. And yes, sometimes you can buy a coin/token for cheaper if you use USD instead of BTC, and vice versa.

Need any more clarifications? I couldn't quite understand much what you're trying to say so hopefully I've answered your question correctly.