We used a double with our then 2 1/2 yo ds and 6 mos dd. It was perfect! But we are going again and now they will be just shy of 3 and 1 yr and we will be bringing 2 singles (though I may do the double and a single as someone suggested!) We will have 4 adults (our mothers are coming too). One reason for the two strollers is that dd is getting frustrated in the back- hard to see things, hits her brother in the head, etc.
Another reason is naps-If you kids will nap in the stroller, take this into consideration. My kids will definately sleep in their strollers so umbrellas are out for us. The regular size singles work because they are separate- if one falls asleep, the other won't wake them. I can recline them, etc if I have the singles. In the double, if I recline the front, dd gets squashed and she has easier access to her brother! If dd is sleeping in the back, and I try to get ds in and out, it will disrupt her. If you have a side by side, you have to worry about them bothering each other! We have a side by side and if one falls asleep, the other has to be still and not bother them.
That being said, if naps aren't an issue, you just have to gauge what your kids will be most comfortable in, especially if they don't mind being in the stroller!