I have a Graco Quattro Duo, which I really love. It's a front/back but the back seat is up a little higher, so the one in the back can see well. The back seat also reclines fully so when my younger one wants to nap, I can lay it down and let her sleep. It's got lots of basket space under for carrying extras, and a small basket under the front seat for the front rider. I really love it, and I don't have too much trouble getting it around.
Keep in mind when you decide front/back or side by side that sometimes those sides don't fit through narrow doorways, aisles and sidewalks.
My sister bought it for me gently used, so I'm not sure the price. I love it, it fits in trunk and I can go anywhere and everywhere with it. I've seen several others when I've been out. Other than my own style, I've seen a lot of sit n stands being used. I don't know anything about them, but I've noticed a lot of them, especially when one child is older and would want to walk sometimes.