I happen to be an IB biology teacher and I think I can give you some direction for you extended essay.
First off, I commend you on reaching out for help developing your question and the way you handled the negative comments left by other users on this thread. There is no shame in soliciting help, especially when you are in the early phases of research.
To all those who left negative or belittling comments, good job! You attempted to internet shame a teenager seeking more (general) information on a topic that they are genuinely curious about. Nothing like a group shame to demolish any shred of intellectualism that managed to survive our educational system.
As to you Peter, no one is born as a gifted academic person. It is true that some academic pursuits come easier to some than others, but in order to excel at anything one must put time and practice. The more you write (and solicit feedback on effective writing), the better a writer you will become. I am sure once you complete your extended essay, you will be a better writer and researcher than when you began.
Anyways, on to your extended essay! In terms of the two question you put forth, the second one offers a more clear link to how most research is currently done and offers fewer compounding variables (performance in rugby is a very amorphous term). However, it's important to keep in mind that part of the EE marking scheme is personal interest, so make sure you are able to make that connection with whichever question you choose.
The other posters were correct in that there is little to no peer reviewed research (that I have seen) to suggest that you can train for one type of hypertrophy versus the other. Your question(s) require some revising in order for you to make an effective argument supported by research based evidence.
I think you should dive a little deeper into the research on this topic before you settle on comparing the two types of hypertrophy in this manner. I admit to being unfamiliar with this topic, however a quick glance at the wikipedia page yields a suspicious lack on citations around your questions. (To the forum bandits, no I am not advocating using wikipedia as a source, however it is an excellent place to find links to primary sources and research or lack thereof.)
As I see it you have two options:
Change your focus and do a research review on the very idea of sarcoplasmic hypertrophy versus myofibrilar hypertrophy. IE: To what extent can an athlete train for sarcoplasmic hypertrophy versus myofibrilar hypertrophy?
Change your focus to some other aspect of training to improve performance in rugby or compound lifts. It seems like you might have been thinking about training in different rep ranges and their effect of athletic performance. There is a plethora or research on this topic, so if you choose this route, make sure you have a very focused and specific question.
My number one piece of advice is make sure you do your homework when it comes to research. A paper such as the extended essay should not just be summarizing and regurgitating a bunch of existing research. It should be a synthesis of a wide variety of research that comes to a novel conclusion answering your initial research question.
You seem to be off to a good start and I once again commend you for choosing a complex and interesting topic. Feel free to send me any more questions you have or if you want me to look at any of your drafts.
For other forum readers who want to share the insight: The extended essay is a capstone or thesis essay that is supposed to be a college level research paper. It should feature a polished, specific research question and use a wide variety of peer reviewed sources.