As you're going for the same make, I'd be inclined to trade it in. If it is in good nick, they should be very interested in it. They also have a strong incentive not to make you a derisory offer, as all that would say is that the new one'll be just as bad. The problem with a lot of trade-ins is that the place you're buying from has no interest in it (ever seen a Hyundi in a Merc garage?), and they have to shift it on to the trade.
In theory, you are better off selling privately and haggling like mad for a discount on the new one, but it's a lot of effort and there are risks involved.
By the way, I'd still haggle like mad with the trade-in: try and go to more than one dealer and play them off one another.