2 stroke vs 4 stroke?

I am getting a Dirtbike this summer and it is going to be my first. I am 16 5'9 170 pounds. I am looking at ether yz 125 or cr 125 for the 2 strokes or yz 250f or crf 250 for the 4 strokes. I am riding mostly trails and sandpits! I am unsure about having to mix the oil every time. Also I don't make very much because I took out a loan from my dad for the bike. So I only have 10 bucks a week for gas and mantance.