I've never seen a performance v cost comparison between the newest F-15 variants v the F-18 Super Hornet, but my guess is the F-15 is still the better airplane. Unless you want to land on a carrier.
The F-15 will be better in a high altitude high speed fight, but once it slows down a little bit the F-18 will have the edge. Thats just my take though.
We need them both of course, they fill different roles.