I think the answer to this question varies depending on how you will be using the stroller.
I found a double snap n go to be absolutely vital for running errands with a car. For me it was hard to get the babies in and out of their car seats when they were very young. Well not *hard* exactly but a chore that I preferred not to have to repeat more than was necessary. They were so floppy and oftentimes had fallen asleep in the car so if I had to take them out of the car seat when we got to the store I would have had to wake them up. With a double snap n go all I had to do was take out the car seat and set it in the stroller. No waking babies up and no buckling/unbuckling and taking in/out of car seats. This way I didn't mind making several stops -- hitting Target *and* the grocery store, for example. Without the double SNG I probably would never have gotten out of the house.
If you don't plan on driving much but will be taking walks out (to bring your older child to school or to the park for example) then I think it would be nice to have something with a bassinet feature, like this:http://www.tinyride.com/Mountain-Bug...wd=GG2057-0078
Obviously that is only for when they are young. Maybe not worth the expense unless you can find one used.
For the longer term/when they are older I find the side by side models easier to push (vs a tandem), but it's true that it's harder to fit through narrow shopping aisles with them. You will fit through most doorways because they are pretty much all designed to fit through a standard handicap accessible doorway. With a tandem the one in the back often kicks the one in the front, with a side by side they squabble and grab each other's hair and steal toys instead.
By far my favorite solution (though not always practical) is to have one in an ergo or similar carrier on my back, and the other in a shopping cart or a single stroller.
Bottom line my best advice is to buy the double snap n go (used if possible -- should be ~$50 if used, ~$100 if new), then WAIT on any further purchases until they are older and you see how you will be using your stroller.