The stock cooler will likely be fine if you're not putting it under 100% load. The big issue will be noise or if you run in a very hot climate. The M9i does do a much better job cooling and significantly better noise reduction, so if budget allows for it I don't see any reason not to get it.
The DeepCool Gammaxx 400 is also a good cooler. With some benchmarks it seems the M9i out performs the DeepCool, and the M9i has what I think is a better mounting system. But really they're both going to do you about the same amount of good. I'd go for whichever is cheaper or easier to get.