You would ordinary take the 310-035 SCJP 1.4 exam.
The other exam (212-035 SCJP 1.4) is part of the Sun Academic Initiative (SAI) and is only available for students and instructors belonging to a school registered in the SAI program. In this case you would need a special SAI voucher that entitles you to take the test paying only 40 USD, instead of 150 USD which is the price of the regular exam.
Another difference is the certification kit you obtain once you pass the exam. The SAI (212-035) certification kit only includes your certificate and a congratulation letter. The regular (310-035) certification kit includes the same things, plus a pin, a wallet size card, and a license agreement for using the SCJP logo in your personal presentation cards and stationary.
subject: Difference between 212-035 SCJP 1.4 and 310-035 SCJP 1.4 Certification