They perform very similarly. My suggestion is to get the one that works best for your budget. Keep in mind they use different sockets, so you'd have to get a socket 1155 motherboard for the i5 3450 and a socket 1150 motherboard for the i5 4570.
Additionally, if you want to upgrade to a new processor within a year or two then you should get the i5 4570 as socket 1150 will be compatible with the next Intel CPUs, Broadwell. Though I would argue that for the average gamer the CPU won't need an upgrade that quickly.
More technical:
i5 4570: Slightly faster stock clock rates and slightly better IPC, making it overall faster, but not by a lot. No OverClocking at all.
i5 3450: +400mhz limited OverClock is possible, meaning you can easily give it a mild overclock to 3.9Ghz compared to the i5 4570's 3.6Ghz. You will need a "Z" chipset motherboard to do this. (Z75/Z77)