I agree with Natalie. I bought a great "Take Me Too" stroller when my second was a few months old. It really was great. My oldest was just over 2 yrs when #2 was born, and we used it until he was about 4. But we just had baby #3, and my very large (>97% for height, 85% for weight) 3 1/2 year old is already a few pounds over the weight limit (40 lbs). I wish I could buy a sit 'n stand, because he still gets tired and LOVES to ride in a stroller. Our Take Me Too is rather useless now. I don't know if they'r still making them, but they're awesome. There's a step for the child in the back to get in by themselves, and the back seat is higher so the back child can see. It won hands down over the Duoglider in my book!
My friend had a side-by-side, and she couldn't get it through our front door. I had to open our double doors to let her come in without taking her babies out of the stroller! She said it was pretty useless in stores, too. I tried a Combi side-by-side at a yard sale, and it didn't steer AT ALL so I didn't buy it.
I suggest that you go to Baby Depot inside Burlington Coat Factory if you can. They have floor models you can try out. Put your son in the front seat when trying out the in-line strollers, because they're harder to steer when there's weight in the front. I was surprised how difficult some of the strollers were to steer, even while empty. It made our decision very easy after we tried them out!
Oh, and as for whether a double is worth it, YES!!! I remember going to the mall by myself to let my B. play before I got the stroller. My 2 yr old got tired on the way out of the mall, and was sobbing for me to carry him. My infant was in his infant seat which snapped onto the stroller. For a while, I was giving the 2 yr old a piggy back while pushing the infant. Then I finally had to take the baby out of his seat, balance the car seat on the canopy (not steady!) and let the 2 yr old ride while I carried the baby. My husband didn't want me to get a double stroller, but after that horrible experience, I said to heck with what he thinks! It was wonderful after I bought it.