The backpack vs rolling luggage debate in budget travel has a clear answer based on specific conditions rather than personal preference.

Rolling luggage wins when: you are primarily staying in cities, all your accommodation is within a short walk of transport connections, you are in countries with smooth paved surfaces throughout, you are not doing any activities requiring hiking or serious walking, and your trip is 2 weeks or less.

Backpack wins when: any of the following apply: you're navigating cobblestone streets, stairs, or irregular surfaces; you're taking overnight buses or trains where you need to store luggage overhead; you're doing hiking or active travel; you're moving between multiple types of accommodation including hostels with luggage storage stairs; you're traveling for more than 3 weeks.

The practical test: if you're going to Southeast Asia, the Balkans, Latin America, or anywhere with cobblestones, a rolling bag is a liability. Cobblestones destroy wheels and the noise is antisocial in early morning hotel lobbies. Dragging a rolling bag up 4 flights of stairs to a hostel dorm is genuinely difficult.

The 40L backpack compromise: large enough for 3-6 weeks of travel, small enough for carry-on, rigid enough for urban walking, flexible enough for everything else. This is why every experienced budget traveler converges on this size.