I would choose the Mustang because it is slightly smaller and thiner in girth, not only that it comes in a wider choice of color combinations. I thought the Tie-bright colorful one looked fun. Also, the Buck VixSkin in beige looked shiny and fake, not as realistic at all. I have the 7" Fetish Fantasy Vibrating Dildo and at first found it a little tight inside, thinking back the 6" probably would have sufficed which is why I would select the Mustang over the Buck VixSkin.
Both are made of the same material, have the same safety rating and maintenance so yes I would spend the extra $15 on the Mustang besides they're both 25% off right now so even if you got the Mustang it would still cost less than the Buck at full price.
One other reason though really not a comparison between the two dildos.... I could say we were a 2 Mustang family, the car and the dildo!