Best use of budget for first time backpacking? How far can it stretch?

I think both are great options, and both are definitely doable on a budget, so it really would be ultimately which you are most interested in doing! For backpacking SEA, the longevity of your budget depends on a few things. When I backpacked SEA in 2018, my whole budget for the entire trip broke down to be about $1200USD a month. I traveled fairly slowly, which saved money because obviously the more you are traveling within SEA the more you spend on transportation. However, I mostly stayed in private rooms, inns, and homestays, etc. since I was working remotely and wanted to have more of my own space. So it would have brought the budget down significantly if I had mostly stayed in hostel dorms. For reference, my accommodation budget was an average of $15USD/night. So if I were to have dropped my nightly accommodation budget to say $8USD/night, I would have saved a couple hundred USD/month just by doing that. I also had a food budget of $9USD/day, which allowed me to eat out for at least two meals a day and be pretty liberal in getting whatever I wanted to try. However, you could definitely bring that budget down significantly by meal planning, cooking your own meals, and sticking to the cheapest options you find when you do go out to eat. You could also bring that budget up if you are a serious foodie and want all the things! I haven't done the Australian WHV so hopefully someone can give better insight in terms of comparing the two budgets, but I hope this was helpful and please let me know if you have any other questions about my SEA budget!