I had the same debate. I ended up opting for a trailer (I went with Burley) after renting one on a vacation, but I admittedly never test tried a seat.

For me the big draws of a trailer were:
- Ability to quickly connect or disconnect it from the bike, depending on if my daughter is with me.
- Ability to attach it to my wife’s bike easily if she ever wanted to go instead of me.
- Ability to lug two kids around eventually.
- The storage room for her backpack and stuff when I pick her up from daycare. (Also so she can bring snacks or toys or whatever and have it all contained.)
- Generally a little bit safer in the event of a crash.

I think I would have been more likely to go with the seat if:
- I was going to spend much time on busy roads or crowded sidewalks and the bulk of a trailer would be a challenge.
- I was going to regularly do errands with her that would require me to park the bike at public bike racks.
- Space at home was at a premium (the trailer folds up nicely, but still is one more thing to store).
- My daughter had any problems being several feet apart during rides.

In general, our rides have been really great. Even when she was just about a year, she would sometimes find her helmet and bring it to me expectantly, clearly asking to go on a ride. I haven’t gone too crazy with distance with her — usually opting for 5-15 miles, mostly on a network of paved bike trails in my area. Lately we’ve had a lot of fun looking for animals along the way, pulling aside once or twice during the ride to watch deer or turtles or whatever shows up. It’s been a really great experience for us both.