Any difference in the tests themselves is corrected by the scale (which is made with the data from the past few years of test performances) - the myths about avoiding the June/Sept test because all the 'overachievers' take the test then are false.
I found the June test to be the best time for me for the following reasons:
1) It would give me ample time to study (unlike the Feb test - since I started studying about this time last year). Although I did have my final exams in May, I was able to study full time for the three weeks prior to the exam.
2) If I screwed up in June, I could retake in September and still get my applications in early.
3) 12:30PM start time is better than 8AM (although I would consider this a minor reason)
Your personal circumstances may vary - there's no one best time for everyone to take the LSAT. But do keep in mind that many schools do not accept scores from the Feburary test for admission that year (for example, if you are applying for Fall 08, the last test many schools will accept the score for is Dec 07). Taking the Dec test for admission the following year will be accepted for admission, but it may put you at a disadvantage since you won't be able to complete your application as early as you could if you took the test earlier.