Neither one nor the other. The idea that duration is necessarily linked to the low season for budget reasons ignores the complexity of travel choices, it's like ignoring the menu to only see the price.
Here in Japan, we know that fine management of daily expenses, like choosing local trains or home-cooked meals instead of restaurants, offers much greater flexibility than just duration or season.
You can very well travel longer without breaking the bank, even in peak season, if you're ingenious with small daily budgets, rather than following this predetermined path.