I would suggest that there is no need for you to upgrade to 6.0 as there is not much difference in scjp5 and scjp 6. If you think you got low score then you can work on the areas where you scored less. Would you want to again give certification for the same subject just because you will confident?
why waste money?
You should now look towards new technologies as in SCWCD, SCBCD.
The differences between the 5 and 6 exam are trivial and unimportant. (Unlike the 1.4 to 1.5 exam which covered important and useful information). Forget about upgrading, do something like the SCWCD instead. Your score was good, don't worry about the score.