Mecheng, you're not doing anything wrong. Both browsers are different in their own ways. I personally prefer the BB Browser because I feel that it is a little quicker than Opera. However, there are some web pages that the BB Browser can't read (MySpace, etc.) that Opera can.
I think it's mostly personal preference. Some people absolutely love the Operamini browswer. I keep the Opera browser to use for the web pages that I can't ready with my BB Browswer. Also, Opera sometimes makes certain web pages much easier to read.
Experiment and see which is more to your liking. Or keep both.