What is the difference between 250W band heater and a 200W one? Can they reach the same temperature?
Heating elements generally have a maximum operating temperature above which they'll start to degrade and eventually break. If these heaters are from the same product line they are likely made of the same materials and will have the same maximum operating temp therefore.
Separately, you want to know how hot they can get in your application. The hotter they get, the faster they lose heat to the environment. Once they are losing heat as quickly as they generate it, their temperature will stop rising. From that perspective, the 250 W heater will reach a higher temperature if all other things are the same in your system. Alternatively, you may add insulation to the lower power heater to achieve the same final temp.
The last thing to consider, is that the higher power heater will heat things up more quickly.