A wheel isn't just a fun toy, it's a necessity. Hamsters need exercise to stay healthy and not fat. Without an exercise wheel/ball your hammie may get overweight and sick, and even die! Hamsters will run up to 8 miles a day in their wheel! Isn't that amazing?! You wouldn't want to deprive them of that, now would you.
       For a tank I recommend just getting a big stand-up wheel. I've tried putting the ball in there and it doesn't really work too well. Buttercup chewed the stand so it practically broke, and they take up a lot of room. However, you should still get a ball. You can take them out of the cage and have hammie roll around on the floor. For a wire cage you might want to get an attachable wheel instead (most wire cages come with them). These can just be attached right to the cage wall and provide another great way for hammie to exercise.In a play cage, you can just buy a plastic wheel which will attach to the side of the cage. I wouldn't recommend a metal wheel, because the hamsters feet can get caught and result in a serious injury. However I use them for my dwarves (it's the only kind I could find) and they seem to work OK-- I haven't had any injuries. If you want a metal wheel, get one with wire going both ways in a crosshatch, so the feet can't get caught. If you can't get one like that, take wide tape and wrap it all around the wheel both ways, it can work just fine.