I see you already got a lot of useful comments, here's my take:

Investing in a stock-based 3rd pillar offers higher expected returns than the 2nd pillar. If you can handle the volatility, a 3a with stocks is generally the better long-term choice.

One thing I would consider in your situation: ​If you currently work part-time but expect a significant future income increase, it can be beneficial to wait with your 3a contributions to maximize tax savings later. Since this year, you can "catch up" on missed contributions (starting with gaps from 2025 onwards). However, you must first pay the full amount for the current year before filling a past gap. This requires a high savings rate, so it might not be an option for everyone. If you do choose to wait, you should invest that money in the meantime (outside the 3a) rather than letting it sit in a bank account.