Personally I like fall better. Hiking paths are not as muddy from winter and the weather is a little bit more dependable It also depends on how late into fall you want to visit. My favorite is late September/early October.

I do agree with startpaking that 5 days in Luzern is too much. While Luzern is nice, the OB area is so beautiful and offers so many more activities...mountain railways, cable cars, hiking, waterfalls, walking on a glacier and spectacular views from the mountain top peaks.