The Enzo will fluctuate in value over the years, but in the long run, will always be collectible and a better investment that the Superleggera and the 599 GTB. The Enzo is also a lover produced vehicle in terms of numbers than what you want to trade. Don't forget the super car pack as well. Many people as time goes on, may decide they want to collect the whole super car group of Ferraris which consist of the 288 GTO, F40, F50, and Enzo. If you have the missing piece to someones collection, you can name your price almost at that point. In short, I would obtain the Enzo all day long over keeping the 599 and SL.
A Veyron is also a great option, I think the Enzo would be a better bet just based on the super car collection like I said above. How many variations of Veyrons are there now? How many variations of Enzos are there? 1! Just my 2 cents and that's all it's worth.
Good luck!