Your question has two answers.
1) If everything really is equal, then resistance is precisely the same as weight. Force is force, whether it comes from mass being affected by gravity or from tension on a spring or from hydraulic pumps or from a ray gun.
2) What Yevgeniy's answer gets at is that it's ridiculous, within the context of strength training (or lifting of any kind, even bodybuilding) to suppose that "all else is equal".
The resistance of a machine exercise is force in a certain plane. In the case of Bowflex-type machines, the force may change along the path of the exercise. With free weights (barbells, kettlebells, dumbbells) the only things involved are the weight, gravity and how your body is structured (your anthropometry).
Let's use your example in the comments of a "thighmaster for the chest" versus free weights. As I understand it: The resistance of the spring would be least in the "open" position and greatest as you get near the "closed" position. If you used dumbbells (say, from a position on your back), the force (from gravity) would be equal throughout, but your body would be applying force in different directions across the movement (always "upward", but using different muscles since the weight moves across different angles in relation to your body), in addition to stabilizing the weight in three planes during the entire exercise.
If you're asking about when to use one versus the other, see this question.