I've always gone with the first one (EEcomodder). I guess that's because it's similar to how I've always heard "eco" pronounced in other vehicle context. For exaple, Ford's EcoBoost, the Chevy Cruze Eco, etc. Then again, maybe other people don't pronounce those "EEco..."
A while back I saw this article, which I thought was pretty interesting. It's maps of the US showing linguistic differences by regions. Things like pronunciation of "caramal", "aunt", "crayon", "garage", "pecan" etc. as well as other expressions such as "garbage can" vs "trash can", "mow the yard" vs "cut the grass", "frosting" vs "icing", etc.
50+ Fascinating Maps That Show the Difference in the Way Americans Talk | TheBlaze.com