SCJP is of course more difficult, but I enjoyed preparing and passing SCJP!
Here are the main differences in my opinion:
1 ) SCJP is focus on understanding concepts and internal behavior of java, and test 157 is focused on memorizing menus, actions, settings, views, perspectives and less on the comprehension of the product. Preparing for SCJP will sharpen your skills as java programmer, preparing for test 157 will make acquire some knowledge of the product, much more you will learn from experience.
2) SCJP objectives of are precise and concise. Test 157 objectives are very general, so you can expect to be asked questions about any combination of menu/view/perspective related to the objective.
3�)You can find very good books to walk through the SCJP certification. You are on your own in preparing for test 157. Of course there are Redbooks, and online- help, but no specific guidance to prepare for the 157 test.
4) You can get mock exams on the net as closely as possible to the real test, and the score you obtain on those won�t differ very much from the real test. The single mock exam that you will be able to find on test 157 is the ICE test with an unchanged set of 58 questions witch is to different from the real test, and the score that I obtained to the Ice test is not even close to the real test.
I hope that my post will help you. Good luck !