I can offer an opinion since I train alot, but bear in mind that I got into the game a little too late to hop into the cage so to speak.
"I just wanted to no what one will be more useful for in the ring fighting"...
MMA and Muay Thai are different sports from a competition standpoint. If you go into a MMA match with just Muay Thai you will probably get owned. I say this because I train with kickboxers who school me until we do an MMA round then things get reversed because they have no, or very little, ground game.
Ultimately it depends on what type of competition you want to do. Again, this is my opinion based on my experiences and there are alot of people on this forum with more than me.
Hope this provides some insight..