Considering whether it would be better to stop in Chamonix in May or September? Essentially, in which of my 2017 French trips should it be included? One trip will be the first half of May, (April 30 to May 16). The second flight isn't booked yet but will most likely be in the last half of September (and/or early October). Primary purpose wouldn't be hiking (I'm not planning to pack suitable hiking shoes) but I would like to see the top. Will it all be open either time? Is there a time of year when it's more likely to be clear? Cable cars more likely to be closed for winds?
I think my answer is going to be September, my experience in mountains (Sierras, Rockies, Cascades) is after summer is better than earlier in the year. Since I've never been in this area, however, I thought I'd ask.