The i5 by a decent margin. The FX has very poor single core performance compared to the i5, AMD tries to make up for this by having better multithreading, but the results have been rather poor on most software, especially when most software only uses 1 or 2 threads, meaning 4 out of 6 cores on the AMD CPU aren't used by many programs. Only a small selection of workstation applications work well with the FX's multithreading, and even then only the 8 core models are really able to compete with the i5s in any serious way.
If you go for intel, you may want to spring a bit more money for the 2500k, and get a p67 or z68 board. The 2500k has a very high overclocking potential, and can give you even more performance if you need it. The 2400 is a locked CPU, it can only OC about 400MHz above the stock speed.