Turntable. You will recoup a lot more for your Debut Carbon than you might for your cartridge.
If the goal is to sell what you can get the most for, then maybe. I thought it was to get the most sound quality? I would hang onto the cart in order to have a virtually new cart to sell with the table. Most people at this price level are looking for a complete package. The table will be worth more as a complete package with the original cart.
You will need to listen carefully as any potentially huge improvements on your Pro-Ject deck will be hard to find.
So, purely as a learning experience, does it really make sense to recommend upgrading a turntable, then immediately saying but it probably won't be worth it as any improvement will be hard to find?