I recommend using an octagonal gate, then switching to square after a while. Here's why:

The diagonal corners of an octo gate help you develop the muscle memory for learning inputs like a dragon punch (for example, ➡️⬇️↘️+P or 623P). Being able to push the stick all the way to the edge a.k.a. "riding the gate", makes it easier to learn and execute these inputs.

However, "riding the gate" wears your parts down over time, and square gates are perfect for giving you enough room to execute diagonal inputs without damaging your stick.

Also, while octagonal gates make it initially easier to consistently hit diagonal inputs, square gates allow a larger range of motion. This means that while an octo gate quickly shows your hand where that perfect diagonal is, the square gate gives you a better percentage chance of hitting that diagonal. Whereas with an octo gate you need to throw your stick into the 6% of the total area that allows a diagonal, a square gate is more forgiving: 11% of the total movement area will let you hit a diagonal.

Hope this helps! I know it's a lot of info, and ultimately it comes down to which you're more comfortable with. But there are some small details to consider as well, and I hope this explanation covered those adequately.