The M2 Pro supports one additional external screen over the M3, has an additional fan meaning less fan noise overall and somewhat worse battery life than the more efficient M3. So if you want to plug in two external screens you'll definitely want the M2 Pro SoC, or the M3 Pro SoC and not just the M3. At 500GB storage the M3 model might have the faster SSD, though that might be limited to the M3 Pro and Max versions, not sure about that. One USB port less on the M3 as it doesn't have the required bandwidth to drive 3 ports, that needs a Pro or Max chip.
If they both got the same RAM and storage then for the same price the M3 has the better battery and if battery life is important to you it might be the better choice for you.
Performance is similar, older Pro SoC vs newer entry-level SoC. There isn't a clear winner here since you aren't comparing apples to apples, the clear upgrade over the M2 Pro would be the M3 Pro. The biggest differences aren't performance, they're the mentioned differences in ports, battery life, monitor support, fan design.
Personally I wouldn't know which one of the two to choose either, I wouldn't want the worse battery but I'd also not want to give up a port or have a noisier device. Apple is great at removing just one small feature here and there until you want to spend more money. If the M3 base model was another 100 bucks or so cheaper it would be an easy decision in favor of M3.