From 2004-6 I lost 20+lbs from my pack.
Over the last 6 months, I lost 20lbs from myself.
Without question, losing bodyweight was the more advantageous by far. If I had to add 20lbs to either my pack or my body, I would put a cannonball in my pack in a heartbeat.
Granted, I was about 30lbs over my "doctor's office" ideal weight. The life impacts of carrying unneeded pounds around cannot be overstated; I seriously had a way lower quality of life. I had lower lung capacity, I slept poorly, I trained less frequently, I sweated and wheezed on the trail. It took me 3 weeks to gain the fitness that I can now gain in a week. My knees hurt, my confidence was lower, and I was seriously drained by the time I got to camp a lot of times. (It actually got dangerous on me when I was solo in the snow last winter; I was too drained to keep myself warm.)
The insidious part is that all of these effects came on so gradually that I didn't even know about them. It was like boiling a frog; I moved in with my gf and stopped my bachelor diet and everything went to pot so to speak.
*That said*, the other time your body doesn't work right is when you're undernourishing yourself -- i.e. dieting. I probably wasn't doing it the way a nutritionist would have advised, but I suspect that a lot of people don't. I skipped meals, I ate lightly, and I used coffee as a breakfast/lunch substitute a lot. During this time, I also had a lack of energy and a hard time building real fitness or endurance. (Of course.)
So I've learned to eat! I get plenty of delicious carbs every day which keeps my (naturally high) energy level high. Then I go out and *use* that energy -- on the trail, even just walking along the seawall by my house for 6 or 8 miles. That keeps me reasonably low-fat (If I was milk I guess I'd be 2% right now; no homo jokes please) while keeping my muscles well supplied with nutrients and fuel!
That's my layman's metaphor-laced rant about weight loss. Highly recommended if your life can tolerate it.