This is one of the most frequent asked questions and there is no RIGHT answer. It really just depends on your comprehension or familiarity of the the exam topics. Going into the exam, I was encouraged to take civil, general, and environmental by 4-5 different people all with civil background and who all passed the exam but I stuck with my gut and chose civil. I chose civil primarily because I had a good educational foundation in hydrology/hydraulics and soils; in addition, I had several years of professional experience in both these areas and some experience in transportation prior to taking the test. So it made more sense for me to go with the Civil Afternoon exam. However, I've heard compiling arguments for each.
If you don't have an affinity towards any of the exam topics or you don't consider any of the topics covered in the Civil Afternoon exam to be your strong suit, then I would probably recommend that you take the General Afternoon because it is basically an extension of the morning exam with more involved problems. By going with the General Afternoon, it's 2-3 less topics (water resources, soils, & transportation) that you have to refresh yourself with if it's been several years since you were last in school. However, if you're like me there were some topics covered in the morning exam (i.e. Electricity/Magnetism, Chemistry) that I hadn't seen in 5-10 years and had NO exposure to since taking the courses so I was trying to avoid additional problems in these topics like the plague.
Best advice is to first determine or evaluate what exam topics you feel most comfortable with or have the best understanding of and make a determination based on that. If you have a good comprehension and knowledge with at least 2 out of 3 of the topics Hydrology/Hydraulics, Soil Mechanics, or Transportation, then I would probably go with the Civil Afternoon exam.