You really need to try several models of each yourself to see which would work best. Some people swear by the side by side strollers, but I absolutely *hated* all of the ones that I tried (including some very expensive ones like the City Mini). On the other hand, there were several tandem strollers (one behind the other) that I loved. My two favorites are the City Select and Contours. We ended up with the Contours because of the lower price and it is awesome. My little one's favorite position is facing each other, but each of the seats can be either facing forward or backwards. We also did look into the sit and stand, but my older daughter was 2 1/2 at the time, so it didn't work very well at that age.
Still, it's a very individual choice and what works for other people may not work for you. If you can, go to some stores, put your kids in the strollers (very important as that really can make a difference) and try a bunch of them out. Also think about how much room you have in your vehicle for a larger stroller, how heavy the stroller is, how much storage room you need in the basket, etc.
Good luck!