I'm not the most experienced backpack maker, but I design fabric stuff for a living, and have tried various software/techniques. I honestly like a mix of hand-sketches, 3D models, and 2D pattern layouts. Software is just a tool: There's no point in spending 20 hours on a perfect model when you still have to recreate the whole thing in real life (unless you're designing something for a customer or whatever and want a good visual).

Sketchup is a GREAT tool: Free, fast, lots of online tutorials, great for quick modeling for overall shapes and proportions. I'd recommend downloading a desktop version instead of the web versions. Downsides: No true circles (24-segment circles, annoying), no to-scale PDF printouts unless you buy a pro license, I think. But you can turn this into this pretty easily by laying out the panels flat in the software.

Adobe Illustrator/Inkscape (free knockoff): These are popular, but I don't have much experience with them. It's just 2D design, but more refined than Sketchup for that stuff. And printable PDF patterns.

Physical patterns: If you're going to make something twice, or revisit it later for an improved version, sure, make a pattern for the complicated pieces and save yourself time later. Tip: Make notes on the actual pattern about your seam allowances and construction, and marks for transitions or hardware placement or whatever.

Combo: Sketch the concept on paper. Do your outer envelope design in 3D CAD (sketchup or whatever). Work out details as you go. Take notes on all the little stuff ("top handle, cut 2"x7" webbing, fold in half, sew on both long edges"). Design your 2D patterns in CAD or whatever. Save it all in a computer folder. Mark a date on your revisions/file names, and your physical patterns.

Have fun!