2500k will be better for quick sync. 8150 even with 8 cpus cores won't able to match it.
if you don't need quick sync, the 8150 would be better for video converting.
http://www.anandtech.com/bench/CPU/28
i only used one encoding test. you can check out how other tests fare. however, the 8150 should consume more power a video conversion load.
either cpu would be fine for multitasking and ms office use.
edit: video editing/converting would greatly benefit from large quantities of system memory. 8-16 gb is suggested. the higher the better. more ram also helps with multitasking and multithreaded workloads.