hheheh.. i can rant all day about this topic ^^" I also was offered a chance to go to several other selective schools (james ruse, syd girls, north syd girls) back when i was in yr 10. I also have friends who've either moved TO a selective school (or even moved OUT of one. That person moved from James Ruse to King's).
The advantages are that you may get better teachers, a better environment to study in which the people around you are more studious (not always the case). Selective school are usually ranked higher than other schools so it may affect your uai.
On the other hand, if you're not doing that well in your studies, you'll get a crappy uai no matter what school you go to. Also, balancing the transition experience as well as school work load, stress, & social life can be a bit hectic. A lot of people fail at this.
The transition into a selective school can also be hard because most are, lets be frank, smarter. So if you do decide to change, be prepared to work your ass off. A warning, not all transition is successful. One of my friend who had the potential to get a UAI of 99/100 ended up getting 89. Wasted talent eh?
Oh...there's a bias towards the disadvantage. hehehe... at this point i should probably tell you that my decision in the end was to remain at my school =D