Can a heater be too powerful for a tank?

I am planning to set up a 28g JBJ nanocube after christmas and was wondering if a heater to have too high of a wattage for my tank size. The tank would be in my room and I like to have my room cold, I usually have my fan on high with windows open and everything. I would estimate my room to be 10 degrees F colder then the rest of my house. Well overall I was planning on getting a 200w Hydor Theo submersible heater and when I looked at the prices I realized that the 300w was only $2 more, so would it be better to just purchase the 300w instead?

 

Couple ways to look at this:

1) If you ever want a bigger tank, that 300 will work pretty easily up to a 75 Gallon (Figure about 4 Watts per gallon). So spending $2 more now will save more later.

2) If for some reason your heater gets locked on, it will cook your tank that much quicker.

3) It will use more electricity (not really an issue - maybe five-ten cents a month more).

4) It might not fit in the back of a JBJ.

I think your biggest issue will be #4 - getting it to fit. If a heater is going to go bad, its going to bad, no two ways around it. For what its worth, I try to always get the biggest heater I can simply because I can move it somewhere else if I needed to. If it fits use it.

 

I think I will just go with the 200w and play it safe