Agree with Janis, I'd choose one or the other. Skye shades it for me but the personal connection has to be a factor too of course. If you do Mull it needs a couple of days minimum I'd say but you could go for up to a week depending on the rest of your itinerary. Book accommodation well in advance as there's less of it than on Skye. Of course that gives the option of island hopping from Oban and seeing as many more of the Western Isles as you wish as well. And yip, impossible to predict the weather here it's just down to luck.

I love Mull, but I'm very keen on wildlife. You are extremely likely to see golden and sea eagles. I remember being taken aback at a notice "TV cameras operate in this area" in a remote part of the island. Of course, they are there to protect nesting sites.
Then, there are the Treshnish Islands for what a boatman called "puffin therapy". You can combine that with a visit to Staffa. Then, there's Iona.
Skye is magnificent, but for me, Mull has the edge