Buying them is a sure thing - you know exactly what you're getting. But it's only one at a time and they're pretty costly. Also, you still don't know if a gold mod with 5 speed will still be at 5 speed or 25 when you finish leveling it, so there's still some risk.
Farming costs you basically nothing, since mod energy isn't used for anything but mod-related stuff. But obviously you'll end up tossing a large percentage of what you dredge up.
Basically comes down to whether you can spare the credits or not. Personally, I mostly rely on farming and whatever I get from mod-rewarding events like Ground War, because I'm forever leveling and promoting my dudes.
