I think it honestly depends. If you can get significantly more for your car selling it privately, then by all means, try and sell it privately. If a dealer decides not to screw you over on a trade in and you live in a state that allows you to pay the difference on sales tax with a trade in when you purchase a new vehicle, then it would probably be wiser to do that. Unless you can get more by selling it privately then you would with the trade in value AND tax break. When I bought my Shelby, the dealer offered to give me $24,000 for my trade in, which was a Focus ST that I paid $25,600 for brand new the year before, so it was a no brainer. It was almost like I lost nothing.
IF I decide to get a hellcat, then I would have to trade my Shelby for it. Whether or not I trade entirely depends on if the Dodge dealer wants to screw me on the trade or not. I wouldn't be making this move until there are charger hellcats on dealer lots, so I have some more time to decide.