1. IBM 486 The objective of this exam is to test your oo analysis and design skills, and checks if you can use and understand UML diagrams for analysis and design. Focus is on applying object oriented techniques. Read http://www-03.ibm.com/certify/tests/obj486.shtml for an overview of the objectives.
2. OMG UML The objective of this certification is to test your UML2 language skills. No questions about analysis, design or Java.
Which one is better? Don't know. They have a different goal. I choose OMG because I wanted to draw attention to my UML skills, and because passing the exam immediately makes you certified.
If the certification is needed to show your design skills then IBM 486 is best to get the desired notice. However, 486 will be discontinued from June 30th. OCUP is best to show that you know UML2 and are skilled to used the UML methodology. Tests 833 / 834 are the ones to take and get certified to have a future in the UML world. Again, take this test only if you have proper experience in this field.