I'll generally start out w/a partial game mod first, both to see how the game reacts to being modded, and to generally get a feel for how well a modder does putting their addition together. You know, drop in a ship here, a mini-pack of missions there, a character skin somewhere else.

I won't do a full game mod unless it's by someone who I've already downloaded some of their other mods that have worked well for me. Another good resource is the people who are on the ball enough to write a 'modding guide' (that works and can be easily followed) for some aspect or multiple aspects of a game, those are the people who I'll trust to download and try some of their full game mods.