trying to decide: mid-late October in Normandy, or in southern France?

I think your weather will more likely be a lot better in the South, but given the choice between those, I'd pick Normandy, hands down. Like you, I don't mind the rain and moderate chill and less people.
What are your interests? We like the history of Normandy and could have spend a lot more time exploring that region.

I have ancestors who left France from the port of St Malo, and my better half is interested in the D-Day beaches/WWII history. I'm especially interested in regional food/baking.

Normandy for the obvious. A must see in my opinion.

Apples to oranges. What do you want to see more? They are so different. It seems like Normandy based on your second post. I'd go to Avignon et al. My husband would go to Normandy.

I'd expect either to be lovely in October.

I'd find out where I'd be more likely to see fall colors.

Both will be off season. I visited Normandie in mid September last year. The weather was nice, some rain, but nothing was crowded - even MSM was only mildly packed.

Provence and the Riviera are likely to have nicer weather. I'm going there in mid/late September this year, so we'll see how it is. I was able to find very affordable lodging, which is usually a good indication it's out of season.

I'd frankly prefer the South (Provence into the Cote d'Azur) in October, as the weather would be better. But if you have familial ties and don't mind damp weather, go ahead. MSM will be wonderfully atmospheric at that time and the WWII beaches and cemeteries are a good pilgrimage. Our friends have a farm near Honfleur and the countryside is quiet peaceful. I prefer the natural scenery of the hills and coasts of the South, and the cuisine, colors and openness.

One trip we were in the south of France, then moved up to Paris then on to Northern France (Caen, Mont St Michel). This would have been last week of Sept in the South, early Oct in the North. The north was def cooler, but still nice. We also returned to southern France the first week of Oct a few years ago and the weather in Nice felt like July does at home. Gorgeous.

For me, mid-late Oct I'd do the south of France, but that's because I get enough cold damp weather at home...