Both are good and work you differently. Endurance rucks for distance at a good pace and intensity rucks with heavier weight for 3 sets of 10-15 minutes at a good pace. If you can do 60# for 4 miles in an hour on flat even hard surface ground and 80# for 3-15 minute sets @ 4 mph your doing good. If you also run and do full body strength training you will have more improvements,
Ray Jardine's Pacific Crest Trail Hikers Handbook might have want you are looking for. One of the 1st chapters is titled "The Pyramid of Hiking Style". It relates hiking with 8 days between resupplies. If you were to hike 17 miles a day at 2.75 miles per hour, you would hike for 6.2 hours per day. If we hike just 1.5 hours longer per day the daily mileage would increase to just over 21 miles per day. At 21 miles per day we would only need resupply every 6.5 day. We could then take out a corresponding amount of food and fuel and hike a little further every day still arriving in camp with equal fatigue. And so on…..Page 26 of Pacific Crest Trail Hikers Handbook. Hope this helps.
JMO, but if you want to be good at rucking you really need a good training program. Upper/lower body strength training, core strength training, lighter endurance rucks and shorter heavy rucks, running or swimming and some type of met-con.....on the rucks you want to work up to 4mph on flat/even ground. I do my training rucks on a treadmill so I have a very controlled environment for progressive difficulty level.
I like to do what minimizes wear and tear on my knees as much as possible. I also don’t have access to hills or any area to get a true vertical miles in. I vary it up by doing different things in a gym like stair master with a weighted pack, running, lunges, squats, burpees, box jumps, step ups and biking in addition to full body strength stuff. I get out year round hunting bears, lions, deer and elk so I never go that long without some hunting with a loaded pack. But I find if I do the gym work, putting pack on isn’t that much of an adjustment.
In my experience, nothing replaces rucking a heavy pack. It should be part of your routine in some capacity, but be careful not to hurt yourself from going too heavy too early. I've seen guys train with a light pack, then complain during the actual hunt because their real world packs are "the heaviest packs they've ever carried". Better to train above the expected physical requirements and have energy reserves, than sell yourself short and train light.
I like to do both as it does pretty well simulate the way I hunt; hunting I typically cover a lot of ground in a day w/ a 20-30-ish lb pack. If I harvest something, then having some training w/ heavier weight (and shorter distances) definitely helps out.
@*zap* is correct, these are just two pieces to the "puzzle", a good strength program and a good aerobic program both positively augment ruck training. I'll add that a day or two of recovery also will positively impact your overall fitness.