If you dont have any other compelling reason like an ongoing project which requires SCJP 5 certified professional or future projects it is always good to go for the latest. there isin't much difference between SCJP 5or6 here are few differences mentioned below.
Comparison of SCJP5.0 and SCJP6.0
Number of questions:
*SCJP 5.0 - 72
*SCJP 6.0 - 72
*SCJP 5.0 - 59% (43 of 72 questions)
*SCJP 6.0 - 65% (47 of 72 questions)
SCJP 5.0 - exam is based on the following objectives:
*Declaration, Initialization and Scoping
SCJP 6.0 - Objectives of SCJP 6.0 are almost same as SCJP 5.0 except few changes which introduce new topics like Console (API Contents),
NavigableSet, and NavigableMap (Collections/Generics) and removed the overridden methods and constructor from "OO Concepts".
Objectives - The seven categories of objectives for the exam are as follows:
*Declarations, initialization, and scoping