I actually floated the idea of selling and replacing with an RCF - it was more about differing priorities with my cars than which car is 'better,' though.
To me, when it comes to a proper/true sports car, E85/86 is one of the best platforms available, with classic proportions. Front engine, RWD, 6 speed only, adequate power, seating is behind a long hood, almost over the rear wheels, all with great handling dynamics. The spiritual sister (with arguably better handling, according to many), would be none other than the Honda S2000!
I also love the idea of an M2/M2C to replace the Z but I fear I'd miss the small/compact feel of the Z4 that adds to the overall driving experience.