How old are the kids? Between 5 months and 1.2-1.5 yrs (roughly) you won't be able to use anything but trailer.
Some seats can be inclined backwards, and kids will fall asleep.

Protection from bad weather is a tricky thing. Our girls won't allow us to cover the trailer with the rain protection, somehow they are still afraid. Or maybe they just hate us trying to restrict their freedom to see the world as it is and not through the piece of film. I find it's better to let them wear the rain protection, just like the adults do. They won't melt. This will do:



If the trailer won't fit, then there is no choice, just go for the xtracycle. I don't know how attentive are the drivers in your area, but if you don't have a good feeling about it, then switch to the sidewalk.

With the seat it's a bit tricky to support the head of a sleeping child. But it's not a huge problem. Trailer is more comfy though, there are plenty of special inserts that make the fit tighter and better. Trailers have better suspension and if the child is little then a hammock does a great thing to smooth out the bumps.

Carrying luggage is probably easier with the trailer.

Trailer is a real pain to travel with trains, buses and alike. On the other hand a trailer often is not just for cycling, we use ours to ski, jog, stroll and sometimes girls sleep in it in the daycare. When the temperatures are well below zero you can use a sort of warm bag to keep the kids warm.





PS. When i was on a parental leave and my wife returned to work trailer was my only way to put both girls to sleep (in hammock). I think i took them for a twenty kilometers ride nearly daily for six month in a row. So trailer was and still is an important part of our day to day life, not just while touring. Wouldn't really be able to do that with seats. But i have no clue what's your situation.