Online proofing saves both time and money, however is not recommended when color is critical. If you are rushing to meet a deadline, then online proofing is a method which will meet your needs. If you are designing for another party, online proofing allows you send the proof to your client via email. As soon as your proof is reviewed and authorized, the print process can begin.
A hardcopy proof is 95% color accurate, and is therefore is recommended for color correctness. By requesting a hard copy proof your order will bypass the automated PDF proofing system and you will be sent a hard copy proof via UPS.