The only issue I see is when the Hydor fails in a few years. Of two heaters I've had fail, one of them just wouldn't stop heating, and I came close to boiling my fish. That was a 300w submersible in a 25g.
Apparently, this is a common result when heaters do fail. And while the Hydors have a good reputation, they're pretty new. There's something about the wiring involving using a standard piece that's prone to failure after a set number of years. Vague, I know, but I read some info on it a while back, and unfortunately don't remember much about it. Do some research - if this is actually true, then the fact that the Hydor ETH has a short history will work against you if you depend on user reviews.
That being said, 1) I use one of them (the 200w) inline and have no complaints after 11+ months, and 2) even if it fails, there probably won't be much difference in the results between a 200w and 300w.