Pick 4.9, unless you really want sound support and some new drivrs for usb. 5.2 is a new technology release so it might have strange bugs(not that i have found any yet been up for nearly 200 days now
) So it is really up to you. If you want a mission critical server pick 4.9, if you want some newer features and dont care that it might crash(never has for me) pick 5.2
In my experience a "unstable" freebsd release are more stable than most stable linux releases