Can you give me insight, do you think trading via ETH/BTC is more profitable than ETH/USD? Please give me an advice.
It depends on what your goal and the current price of BTC and USD/USDT.
If you want to secure your profits, you are better to choose USD/USDT because the value won't change significantly in the future. So, it is safer to keep or directly convert to your account banks. But if you want to sell your ETH for another investment attempt, you can choose BTC. For example, when ETH is rising and BTC is decreasing, you can choose ETH/BTC (selling ETH for BTC). But it rarely happens because commonly ETH price is following BTC price.
It is indeed subjective to the market performance of these currencies.
There's no concrete strategy on how you can trade these to optimize profits.
But if there's time that you don't want to trade and not worry about its fluctuating value, you can always convert it to USDT.
Also, do remember, the other alts are usually following the BTC performance in the market. You may consider such thought.