I am in the same position -- my son is almost three and I have a newborn. I always thought the Joovy Ultralight Caboose (sit and stand) would be perfect for us but when I tried it out in the store before the baby was born my son never seemed to like it. He also didn't like the Sit and Stand Deluxe tandem stroller that has the back seat that removes to become a sit and stand when they're ready. He said he was squished in the Joovy, because when you put the car seat in the front, it does make it a little more cramped for the child in the back if they're sitting on the bench.
What I ended up just getting is the Peg Perego Aria Twin 60/40 side by side stroller and my son seems happy in it. It's light, accepts a car seat on the wider (60) side and just about fits through standard doorways. Peg Perego says it only works with their car seat but the Graco SnugRide fits in it. The store I looked at it in had a few extra straps you can buy to secure most other car seats in. What I've been told by a couple sales people in different stores is that Peg Perego used to include the strap as standard with all their strollers but now you have to call/go to their website and buy one.
Just keep in mind that there is no perfect double stroller out there, from what I can see! Some don't have cup holders. Some are too heavy, the basket is too small/inaccessible. Some are too wide, bulky, what have you. Some don't have enough legroom for the older child, I could go on and on, I've read a lot of reviews!!!! You just have to find one you can live with.
I tried carrying the baby in a sling/Baby Bjorn and pushing a stroller but sometimes it's awkward. And at three years old they may not like riding in a stroller all the time but there are going to be times when their legs get tired and they can't walk anymore. And there are times when I have realized that I need my son strapped into a stroller or else he is going to take off (especially at the mall, museums, outdoor venues like street fairs, zoos, etc).
I just got a great deal on mine at Babies R Us as it was on clearance because it is the 2008 model. And they give you 90 days to return it for any reason with a receipt so that's a good thing.
Good luck!!!