I have many rulers and so does my sister. The one thing I can tell you it's all a personal preference. I have some that I love that my sister doesn't like and visa-versa! I dislike any of the rulers with the yellow lines. That yellow just makes it very difficult for me to line up the edge of the fabric. Wonder why that is? I know many people really like them.
I do appreciate having the small 6 1/2", 8 1/2" and 12 1/2" squares. They make squaring things up so much easier. My 15"X5" rule is great for strip cutting.
Use your coupons and buy a new ruler when you can afford it and you'll soon know exactly what you want! I think rulers are like fabrics, you can never have too many. Scatter them around the sewing area so you always have one at hand!