I think that either way you'll learn all of the new Java 8 features that Oracle thinks OCP 8's should know. I think that I'd do the upgrade, but I don't think there would be a big difference in the end.
Spot false dilemmas now, ask me how!
(If you're not on the edge, you're taking up too much room.)
shuba gopal wrote:Do you advise I do the OCA 8 first or try for the upgrade exam? My goal is to become fluent with all the new features of Java 8
It really depends on your personal preferences. If money is an issue, taking the upgrade exam will be cheaper than taking OCAJP8+OCPJP8. If you don't like certification exams, taking the upgrade exam will require passing only one exam whereas taking the OCAJP8+OCPJP8 path requires passing two exams.
But if you don't object against either of the aforementioned differences, you can choose for either path (upgrade exam vs. OCAJP8+OCPJP8). The OCAJP8+OCPJP8 path will require you take (and pass) two certification exams: one entry-level exam (OCAJP) and one advanced one (OCPJP). Since you are already OCPJP6 certified, you'll probably be prepared rather quickly for the OCAJP8 exam. The preparation time for the OCPJP8 exam and the upgrade exam will probably be very similar since both are advanced exams where you need a in-depth knowledge on a broad range of advanced topics.
PS. Please do not hijack someone else's topic (certainly not with a totally different question). Just create your own topic with a descriptive subject.