It depends on what you want to add weights on your sprints for. If you are a competitive sprinter (meaning, you run track for a club/school/college/etc.), adding weight would probably be a bad idea. Some track coaches have slower athletes run up slight inclines (I believe you don't want to go more than 10 degrees) because that will help getting the proper joint angles that they can't achieve due to lack of strength. However, you generally want to keep the weighted stuff out of your program.
If you are doing this recreationally for fitness or you are another athlete (football/basketball player), weighted runs could and probably will work very well. Obviously in football you can't worry about perfect running technique so that isn't as much of an issue. I would definitly recommend the sled over anything else. I think if you go too heavy with a vest you are setting yourself up for injury and parachutes are just retarded (if there's any sideways wind you're basically fucked). The sled is very versatile as there are dozens of different workouts you can do with it. You can literally get a whole body workout with the sled.
If you're not a competitive sprinter, here's two good things you can do with the sled.
Heavy weighted "pulls"- this is less of a run and more of a pull. Load up a lot of weight and pull it for short distances. The main effort will just be overcoming the inertia.
Light runs- If you are running with it a good guideline for all athletes is 20% of your bodyweight or less. This will work acceleration more, so still keep the runs fairly short (<40m).
If you are looking to do runs of more than 40m I would keep the sled out of it, but that's just me.