It depends on your current experience with Java. Starting with Java 7 it takes 2 exams to get to the same level as Java 6 or before.
OCA7 is for beginners in Java and OCP7 is for more experience people. Java 7 exams cover more topics than Java 6 (eg
JDBC, NIO, localization, etc).
At the end, do you want to get Java 7 cert? Because there are 2 paths: OCA7 then OCP7 or OCP6 then OCP7 upgrade.