I say stick with your RT Classic, unless you plan to sell it yourself. Trading it in for an SRT, you will not get a good deal.
With your current car, you could supercharge it, for around 7K, and it will be faster then a SRT.
The above is definitely the truth.... I would refinance if possible and go with the SC option.
I would also agree with Looking at Ebay, and looking at CarMax (does not hurt to check) I would also check out trucar.com at least for prices paid ( I am not sure on the used values) Keeping in mind that the prices or deals people get avg in your area for new challengers, hurts the trade in value as well.
Currently you have a 3 model year car with 20k miles on it. Going out on a limb here, I would say you are probably safe checking the excellent or really good condition box on the online price guides. Realistically, without being insulting to you and not ever having seen your car, that price is not too bad. Not good, but not too bad. I believe they might have some wiggle room in what price they offer you on your trade, but I would not think they would give more than another 2k on that.
What is hurting you is, you are going from a used vehicle, to a used vehicle, there is not as much markup or wiggle room on either car. If you went for trading in on a New SRT8 Core, I bet you would get a better trade in offer only because there are more incentives and ways a dealer can hide the money you are losing/ paying for the new car. Especially if the dealer is already below MSRP by 5k on the SRT8.
good luck with whatever you choose, either way I am confident you have or will have a sweet car.