مُتَحَمَّس : this is the correct word for it.
The other word is usually the dialect of many Arab people. For example, I am from Egypt and we pronounce it - motahamis.
The one found in dictionaries is the "Al Fus-ha" way, and is the very accurate way to say it. Yet, when you speak to Arabs with many different dialects - you may pronounce it in the second way.
Hope this clarified your point.