I would like both, but I couldn't justify it. Side-by-side is much easier to maneuver. Go to Buybuy baby and put the sand bag babies in the stroller and push it around. I chose Britax B agile because my britax car seat clicks in.
i prefer a side by side. the front/back ones get hard to maneuver esp as they get older (and by older I mean 1 yr old). as someone described it to me (and i agreed once i tried pushing it with 2 kids in it) it is like pushing a bus.
i love the city mini double. its a side by side, i have no trouble fitting anywhere and it is SO easy to maneuver.
for twins get a double snap and go - I would also try to pick up a single snap and go used too - if you have one that is a little colcky or like to be held - it is easier to wear one and push a single
as for strollers- I preferred the side by side - I have two - I have a large one for walking/running with the dog and a maclaren - honestly the easy to fold umbrella one is best- they can't go in the umbrella till they are about 6 months but it is easier to put in the car and takeout and set up -
if you plan on walking long distance get a big stroller but otherwise they are really not worth it - you mostly use the snap and go the first year with twins and then mlaren is easier in the car
when they are a year ask for a wagon for their birthday