And what age?! I'm weighing the pros and cons of both. I read that the baby should at least be a year old?

With a bike seat, I like knowing she's close to me, I can hear her, she won't overheat like she could in a trailer, it seems less bumpy.
But with a trailer, I could fit both girls in there and they're closer to the ground in case of a crash.

Any advice? Or preferred brands?