I'd never buy a double unless they were like a year apart. My son will be over four but he walks everywhere. If anything I'm going to put him in the stroller and wear or carry baby.
Sit n stand. I have the Joovy Caboose. The only downfall (for me), is my 3 year old can't really take a nap in it very easily if we are out for a full day. There is also little storage in the one I have, but I bought stroller hooks so be able to hang bags on it so that is fine.
We have a sit n stand, and I really like it. We don't use it as much now that they are bigger, but with another one coming along, we'll probably get more use out of it.