Should i buy a mountain bike or a dirt bike?

alright this past month i made a little over $1000 and was either going to buy a really good mountain bike to ride trails and get back into shape on or a used rm125 which is in my pricerange. i have found both. the dirt bike is 1300 and the mtb is 1500 top of the line. i might use the mtb a bit more but not much because i have many dirt biking places a little ways away and i have a motorcycle trailer so that is not a problem. If it matters i already have a descent mountain bike but it has been kinda acting up lately.... can someone help me see the light? (i am overweight if that matters)PLEASE ANSWER!