SAT II are not necessarily harder nor easier; it just depends on the student if they are better in one specific subject which helps since the SAT II are SUBJECT tests. I took AP Chemistry and took the SAT II Chem and I did fairly well. But I also did a lot of prepping in terms of reading thru my notes before the test, skimming the book for topics I haven't learned yet, etc. Generally, the SAT II subject test covers the general material as the easier questions and goes more in depth, as the questions get more difficult. Sometimes the prep books are harder but then again, these "hard" questions are still fair game for the test. Maybe not as many of them, but if you can work your way through the hard questions, you should do fairly well on your exam.
As for your chances of getting into UCLA, (just to give you a fair warning) the UCs are very unpredictable in terms of admission, ESPECIALLY UCLA. In my High School, a lot of my friends who earned cumalitive 4.2 and higher, got accepted to Ivy Leagues such as Yale and Harvard but did not get accepted to UCLA. It's not that UCLA know you didn't do well in high school, but UCLA probably predicts (yes, i know it's lame but it's true) who will actually ATTEND their school in the fall. So they might reject you just thinking that they better open up that slot for someone who would actually intend on attending instead of a student who applied but aims and reaches ivy league level.
Also, UCLA considers MANY OTHER factors ALONG with grades during the admission policy. They look at your extracurricular, volunteer, achievements, contributions, personal statement, struggles, ETC! They dont want a smart-a$$ with no social skills/social life, but a well-rounded, outgoing yet intelligent individual that can succeed under the pressure of a new environment and growing up in college.
Just dont limit your options. And if you can do that well in High School, in terms of GPA, try to look into Ivy Leagues/ liberal art colleges. They're actually cheaper since they're rich and more generous with loans AND they provide a caring environment for students all around.