Which would you choose ? Lake Garda or Lake Como if you could only visit one ?

Both are beautiful, although weather may be an issue at the time you are going (very rainy time, on average).

I would make that choice on the basis of the overall itinerary you envision for your trip, which as I said is at a time when weather may not be favorable for the outdoors, and therefore should prioritize visiting cities.

If your itinerary takes you to Veneto (Venice, Verona, etc.) then lake Garda may make more sense as the others are out of the way.
If your itinerary takes you to Milan, but not Veneto, then lake Como makes more sense.
And if your itinerary includes arrival at Milan Malpensa airport, then Lake Maggiore may make most sense of all.

We might be seeing Milan but we are definitely going to the Venice area.... we have a month in total. Just exploring options at this point. Everywhere in Italy looks amazing. And they all have Gelato so no place looks bleak. LOL

Oh and we own a summer business so late October and Early November are pretty much the only time we can go to Europe before heading south for the winter where I take reservations for our business while trying to recuperate after a long season.

My suggestion would be about the same as Roberto's. Your choice will probably be dictated to some extent by your overall Itinerary.

If Itinerary were not a factor, my suggestion would be Lake Como or Lake Maggiore. The mid-lakes area of Lake Como and especially the small town of Varena is a big favourite with many here. I also like Stresa on Lake Maggiore as it's a beautiful location and offer some good day trip possibilities, such as Locarno (CH).

For my taste, Lake Como (we stayed in Varenna) is more beautiful and serene, more easily done without a car. We did not really enjoy Lake Garda as much and did not find the towns as compellingly lovely. Stresa on Lake Maggiore pretty but a bigger town, easy to visit the area without a car.

If the lake visit is at the end of your trip and you're flying out of Malpensa, Stresa is a good choice as it's relatively easy to get to the airport.

Please be sure to check on the seasonal closings. A lot of Como shuts down except for the city of Como, once the summer season is done. Be sure to check the ferry schedule, which also adjusts after the summer season closes.

I also recommend Varenna - very charming without the over hyped and over priced Bellagio.

I lived on Como for two months at the end of the summer season and loved every minute of it. Watching the sunrise and the sunset are gorgeous. The funnicular up to the top of the hill for the views is worth it (not a lot up there but the views are spectacular). The water ferry isn't just transportation -- it is a beautiful way to be out on the water to see the beautiful villas, charming towns and enjoy a sunny day.