For endurance and resistance, rather you can strike a balance between the two. Do high reps, lower weights for one week for all body parts and smaller reps and higher weights the other week or you can do two weeks of resistance and then take a break and do 1 week of endurance (high rep, smaller weights).
I think that is how it is supposed to be which works for me. It is through that once your body gets used to a particular program, you wont get much out of it. You need a variety after every six months at least, not just in terms of heavier or lighter weight but also type of exercise. It is different for everyone. I keep changing my exercise program on a half yearly basis, concentrating on a particular muscle. Do not get attached to a single machine or a specific exercise. Use all of them, machines, barbells, dumbbells, and keep rotating them.
Just remember. You do not want to be comfortable when working out because no pain, no gain.