It kind of depends on what gun you want. For example, they both stock many of the same guns, but one site may be cheaper for a particular gun, so in that case it's the best site.
However, as a general rule I've found Evike have a better range of products, but Airsoft GI have slightly better prices. So if I find a gun on Evike I like, I'll always check GI before I buy to see if it's cheaper.
In terms of customer support, evike is far better, and the same goes for any custom work you need doing.
But interestingly for tactical gear, GI is far better.
Hope this helps