with six days, four in Varenna, should we spend the next two days at Maggiore or change to the village of Como and tour from there?
What direction are you going in after that? Do you want to avoid backtracking?
Como is not really a village, it's a fair-sized city for the Lakes region. You could visit it in a day from Varenna without changing bases.
I might do a 3-3 split between Lakes Como and Maggiore.
3 nights in Stresa (lake Maggiore) and 3 nights at lake Como (Varenna? bellagio? Menaggio?) would be my choice.
I tend to prefer lake Maggiore to Lake Como, as I think there is more to see (Booromee islands, Cannobio, Orta San Giulio on Lake Orta, Centovalli railway from Locarno), so I would probably allocate an extra day to lake Maggiore, rather than Como, but that is just my personal preference (probably not Rick Steves' who seems to prefer lake Como).
I agree with Roberto. Splitting your time equally between the two lakes, or adding an extra day to Lago Maggiore to allow a day trip to Locarno would be a good idea to consider. If you'd like to try that, you can travel by train to Domodossola and then take the scenic Cento Valli Railway to Locarno, returning to Stresa in the early evening by boat. You may also enjoy THIS video summary of the trip, which shows some of the great scenery.
I'd suggest going early to allow some time to enjoy the sights of Locarno, including the impressive Madonna del Sasso Franciscan Monastery or a trip via cable car to Cardada, which is above the city and has incredible views. There's a hotel & restaurant right beside the cable car station at the top, so you can have a fine hot meal or a cool one, while enjoying the views of the lake while Paragliders sail by.
If you stay in Stresa on Lago Maggiore, that's a very convenient location to access the boats that go to the Borromeo Islands. Isola Bella is one of the largest, and has an elegant villa along with ornate gardens (complete with Peacocks). Isola Pescatore is quite small, but a great place to have lunch in one of the restaurants that have patios along the lake front. You could also take a trip on the cable car to Mount Mottarone, visit the alpine gardens and then continue to the top for great views (provided the weather is not hazy).
Pleased to be reminded of my visit to Isola Bella. It was a long time ago but I remember it with pleasure.
I am a big fan of Stresa as well and since my visit there , I keep dreaming of returning . For all those who are similarly affected , have a look at this charming nine minute video http://youtu.be/zCztktfWAIw