side by side
PRO's
older child can 'see' baby
not long as stadium or back and front type prams
slim fold down
widers parcel area

CON'S
older child waking sleeping baby
older child may hit or hurt baby unintentionally
wider means harder to get into door ways / isle in some cases
harder to place into some cars due to width

One behind the other
PRO's
Baby can sleep peacefully
Older child will be busy looking about
long cargo area
can fit into isle and car's easier
Toddler seat is removeable on some (eg Phil and ted) giving you more $$ for money when older child no longer sits in pram
More protection for the baby whilst toddler can have fresh air

CON's
often with the limo style its harder to get into the undercarriage
stadium style...I found that my baby slid down and was positioned awkwardly due to the staps
stadium style - fold up 'thick' for in the car
limo/stadium style prams often have small wheels, bit hard to steer when there is an older child in the front
bit harder to turn especially for walking.
be carefull what kind of toddler seat pram combo you get....ive had a few tip on me

Ive had alot of prams...best on I had in all honesty was a phil and ted..YES they are more expensive but in the long run you will get your money's worth.
Baby at NB stage is snug, safe and secure..Toddler seat sits over the front and in no way can harm the baby but they can still see baby
when they get older you can put the toddler seat on the back for the baby to sit up in and the normal seat for the older child...great protecction all round...fantastic for walking...fits EVERYWERE easily.

Goodluck on your perfect pram quest