i'm a big promoter of taking it after soph year (august)
1) taking it during the school year is hard--you have to balance responsibilities, and unless you are superman (or woman) you usually slack in some area. That way during school you can focus on your schoolwork.
2) by now most of your requirements and material for the MCAT have been covered. IF you have a strong background in physics or bio, or even chem, it is possible to take the MCAT without completing all of your pre-reqs, though it is not recommended
3) Taking it in the summer allows you to focus on it more exclusively (similar to 1) you can offset the "task" of taking the test by doing something you enjoy (being from socal that could be surfing, playing volleyball, etc). This way you can have fun and also take the MCAT.
4) You can still better your CV by participating in summer EC's. I shadowed a Doc and did some research the same summer I took the MCAT. A month before the test, I went light on my responsibilities so I could crack down...or at least try to crack down.
5) your MCAT score is good for 3 years. That way, if you choose to take a year off after undergrad you can still use the same score. More than a year off in my book is a waste of time (unless you are unsure about your fate in medicine), so you have plenty of time to do your apps.
hope that helps!