With 9 days to spare, spend them all in Rome, or also spend a few days in Naples?

The title summarizes my current predicament. I have 9 days (late April, 20th to 28th) to spend between my stay in Florence and catching my flight back to Lisbon in the morning what would be the 10th day.

Currently, I'm leaning towards spending 6 days in Rome and 3 days in Naples, or maybe a 5 / 4 split.

I worry that 3 days might not be enough to get a good feel for Naples and the surrounding area (classic spending too few days in a place and regretting it later), but 9 days in Rome might also feel like too much.

I'd like to know the opinion of more experienced travellers, am I worrying too much?