Well if you're looking to get serious then focus on what you're worst at. If I remember correctly muay thai is mostly striking so that would be good if your striking is weak whereas (if the matches I've seen are any indication) mma seems to be a bit more focused on grappling.
Personally I've never really like grappling, you're really stuck if the person happens to be better than you and you're rolling around on the ground. Kick someone, jump back and wait for an opening. Then kick again. The only time you go in to strangle them is when you know that they can't possibly beat you.
In all of the universe there is only one person with your exact charateristics. Just like there is only one person with everybody else's characteristics. Effectively, your uniqueness makes you pretty average.