The debate about carry-on only vs checked bags comes down to one thing: how long is your trip and what's the climate range.

For trips under 3 weeks in a single climate zone: carry-on only is clearly better. You move faster, you never lose luggage, you can take budget airlines with low carry-on allowances (Ryanair, AirAsia) without fees, and you can take overnight trains/buses without the stress of a large bag.

For trips over a month spanning multiple climates: a checked bag might be worth it once. The calculation: if your itinerary takes you from Southeast Asia (summer clothes) to Japan in winter (cold weather gear), you either pack a bigger bag or buy and discard cold-weather clothing in Japan. Buying a cheap down jacket and thermal layer in Japan then leaving them at a hostel free box at the end costs about $40-60 and is often cheaper than a large checked bag on multiple budget flights.

The bag I've traveled with for 4 years: Osprey Farpoint 40. 40L is the sweet spot — it fits in every overhead bin I've encountered, it has a hip belt for carrying long distances, and the front-loading design (not top-loading like a hiking pack) makes it easy to dig through your stuff.

The one thing I've added that I wouldn't give up: a small packable day bag. Being able to leave the main bag in a hostel locker and walk around with just essentials is worth the 200g of the packable bag.