What's your budget?
GHK M4 are great and reliable, mags are cold weather resistant too, no problems on semi in 5-10 degree weather last winter.
Upfront cost is a lot though.
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WE M4 are alright, going to have to change things a lot more frequent then you would on a GHK M4, but the WE go for about $250-350 used with some mags.. if you are buying new (400-500, just grab the ghk for 600-650).
Mags indoor during the winter sucks hard though. Saw my buddies M4 and 74U barley get through 20 rounds on semi. GG
Good thing about WE you have a lot of companies giving aftermarket support, now Angry Guns is giving some support with their trigger box. Also parts for the WE are widely available cause everyone in North America loves WE.
http://www8.wgcshop.com/wgc2008/main...GY-ACTB-WEM4SV