For your business, I would judge it this way:
20–100 pieces: DTF is usually the better choice.
No screen setup is needed, production is flexible, and it is suitable for small batches, multiple designs, multiple colors, and fast delivery.
100–300 pieces: Start comparing both methods.
If it is a simple design with few colors and a fixed layout, you can get a screen printing quote. If it is full-color, gradient, photo-style, or complex artwork, DTF is still very competitive.
300–500+ pieces: Seriously consider screen printing.
Especially when the design is fixed, the print position is the same, and the colors are not complicated, screen printing can usually bring the unit cost much lower.
Over 500 pieces: For most simple designs, screen printing is usually cheaper.
But if the artwork is full-color, detailed, has many colors, or the order includes multiple sizes and design variations, DTF may still be easier, safer, and more flexible.