Do you have Rick's Switzerland book? He covers Luzern and the excursions from there quite well, including step-by-step directions for seeing Mt. Pilatus and Mt. Rigi.
I only went to Rigi, because I only had three nights (the first was the day of arrival so I was not yet at full speed). I chose Rigi over Pilatus for the simple reason that I had a Swiss Pass, and my travel (boat from Luzern to Vitznau, train up to Rigi Kulm, train down to Rigi Kaltbad, cable car down to Weggis, STEEP walk down to the Weggis dock, and boat back to Luzern) was completely covered. This took much of a day, and was great. If the steep walk is not for you, don't go to Weggis, but return to Vitznau, where the walk between the train and the boat dock is short and almost level.
A Swiss Pass only gives a 50% reduction for some of the transit to the top of Mt. Pilatus, so with my limited time I didn't do it.