I haven't compared performances but they should be comparable in performance. They i5-3450 is a die shrink and would consume less power. There is also better integrated graphics compared to the i5-2400.
Well the i5 3450 will probably give you a 5-10% IPC improvement. It also has a slightly higher turbo frequency, lower power usage and better integrated graphics as mentioned above. I'd say it's worth the $20 extra overall.