OK, we'll start by ignoring the text and focus on the just the image on the right.
In many children's stories (Sleeping Beauty being the best well known) the antagonist consults a magic mirror inquiring who the best/prettiest/fairest/ect person in the land is , and instead of of reflecting back with the antagonist, the mirror shows the protagonist.
So this image is conveying that Mac > Windows (Like we didn't already know that [sic]).
So the punchline as I see it is this: being a dell product, they're essentially saying they work for the "Evil Witch" but they want to be with "The princess".
Add this into the equation:
Dell: Apple should close shop - CNET News
..and it's seems funny (at least to me).