No worries. As far as I know, mainstream and selective classes will be both combined in Stage 6, meaning that choosing either school will lead to the same outcome in terms of delivery.
The more important consideration is the school rank, as that is an indicator of how academically capable a school's cohort is. Since Chatswood High School is ranked higher than Ryde Secondary College, it would likely be the better option overall. With respect to your Assessment Mark, you can expect a significant percentage of students at CHS to perform well in their HSC exams, meaning that you likely won't need to rank extremely high (although that is still recommended regardless of the school you attend) in order to achieve a good Assessment Mark as you would in a lower-ranked school. Of course, the situation is similar with RSC, as you can still perform extremely well at a school ranked 130th, the main difference being that you may need to obtain higher final ranks at RSC if you wish to receive high Assessment Marks.
Regarding selective vs mainstream, while selective classes allow you to experience a more study-focused and intense environment (allowing you to develop a good study mentality and effective study methods, which will be extremely important as selective and mainstream classes become combined and more importantly when you're in year 12), this does not necessarily guarantee good performance. While selective students may outperform their mainstream counterparts, the opposite is also true, i.e. mainstream students may also outperform some of their selective counterparts. You can develop and implement effective strategies to make positive contributions to your academic performance (particularly in year 12) even if you're not part of your school's selective stream.
In conclusion, if being part of your school's selective stream is a priority to you (which of course has benefits such as those I highlighted), then you should consider RSC. Otherwise, CHS is likely the better option overall.
I hope this helps!