When my second daughter was born, we started with a Joovy sit n stand type of stroller, it was aweful! My oldest daughter, 21 months at the time, could not sit on the toddler seat while we had the infant carrier in the front. I returned that stroller. Then I bought a Graco Duoglide. I liked it. It had lots of room for the infant carrier and my toddler. But when I would try to walk at an increased pace, like to exercise, or when I had to hold one child while pushing the stroller, I found it to be very difficult to manuver. So I broke down and bought a BOB Revolution. I LOVE IT! I bought it, thinking that I would just use it for exercising, but since I got it, I have not even taken the Graco out of the garage. You can push and turn the BOB with ONE HAND!
As far as do you need a double stroller? I would suggest one, because if your son ever got tired of walking around and you are carrying your infant because she is crying, nursing, etc. you are stuck unless you have a stroller to plop your toddler into. Plus, I find that my toddler will often "take off" running, so I make her sit in the stroller when she doesn't stay near. Now that both of my kids are toddlers, it is so nice to have a stroller for both of them. I do keep an unmbrella stroller in my car for those times when I know for sure that I'm only going for a short time and just need one of them confined.
Good luck! And I got my BOB off Ebay. It was used but in very good condition.