Charlie Borland's Stroller Equipments

So I have a 20 month old and I have a Lightweight Maclaren stroller if i need it. My husband and I are talking about trying to conceive in a couple months. So If I get pregnant when my daughter is 2 or 2 1/2 then she will be almost 3- 3/12 yrs old when the next baby is born. So would i need to buy a double stroller? or should i get something like a sit and stand?

also I have a peg perego pliko p3 stroller that i used when my daughter was an infant. and It has a foot stand on it in the back for another child to ride. I thought about using that, but if i were to go to the zoo or aquarium or something like that (which is actually alot) standing would be to much for a 3 yr old!
So what do you think is best for my situation?
also if i might add: we do alot of walking! we live in the city. Too much for a 3yr old in my opinion. my dilema is to get a sit and stand or to get a regular double stroller? im confused on which is best..