Pomodoro technique and entering into flow states are two commonly mentioned productivity techniques used by people studying or doing creative work. I have used these two techniques and I feel more comfortable with the flow state is it feels like a hyperfocused state. Are there respectable scientific studies that support these techniques? Which technique is better?
Comparing the Pomodoro Technique and flow state is like comparing tomatoes and oranges. They are not two different techniques that you choose between. Pomodoro is a productivity / time management technique, and flow is a mental state. They are related in the sense that you could use Pomodoro to help get into a flow state, but you could also use some other technique for that purpose, which it sounds like you already do.
The canonical reference for flow is Flow: The Psychology of Optimal Experience by Mihály Csíkszentmihályi, who first used the term to refer to a mental state.
One thing to keep in mind about flow is that it's good for productivity, but not necessarily good for self-improvement. In a flow state, you tend to accomplish tasks without conscious thought, which accounts for the high productivity. But this also means that you aren't thinking about how you might get better. The process of deliberate practice could be seen as an anti-flow technique, where you consciously focus on each step of a task in order to improve. (1)
I didn't find any rigorous scientific research about the effectiveness of the Pomodoro Technique. This HBR article has some informal results, and anecdotally it's a popular technique among knowledge workers.