Sicily or Greece in late September/early October

I think either would be good in late Sept/early October (have not been at that time but would in a heartbeat). What month would you have available in spring? Late May in Greece was perfection (and has been so on most of my trips to Italy in that time frame). In early spring, I would worry about cool weather and rain. But I am more of a heat lover for sure. Both places have so much variety I think you could plan around inclement weather.

My one visit to Sicily was in early May and I found it a perfect time of year for exploring the Greek ruins on the Island. Cool at night especially near the coast. I was in Crete once in the spring (can't recall if it was April or May.) Again the weather was perfect for exploring and walking - the wildflower season was just finishing, but cool at night. Three other visits to Greece have been in late September/October and it was hot (around 90-100F). Thank goodness for air/con in hotels and museums! Rain was never an issue.

We have been to both Sicily and Greece in September through October.
Both were quite warm in September, with much nicer weather in October. Perhaps Sicily would be better in spring as it gets more uncomfortable in the heat from our experience.

Not even close, Greece wins by a mile.

Don't ignore the historical sites and a couple of museums in Athens (The New Acropolis Museum and the Archaeological Museum).

Once in Athens, I took a three island tour that was a day trip from Piraeus. Suggest that one.