If you have been preparing for OCJP6 then go for it. You can always upgrade to 7 later.
As for the major differences between v6 and v7 ... 1) v7 needs 2 exams (OCA7 + OCP7), 2) v7 OCP exam covers more topics than the old OCP6 exam (eg NIO,
JDBC, design
patterns etc)
As a side note: OCA7 is basically a subset of OCP6 + plus some
Java 7 new features.