I'd (as a childless person...) probably go for the trailer option for various reasons:
A trailer will carry on taking the kiddie until they are bigger than the seat will. Also, you can use it when the kiddie isn't in it, for shopping. Having used a trailer, but not a child seat, I sort of assume the child seat raises your centre of gravity a bit. Also, it seems to me it's harder to get the kid in, standing the bike up and all that. And I know that trailers are pretty much a roll cage, and very safe, if the kid is strapped in properly. And people give you more room when towing a trailer. On instinct, I'd go for a trailer, but that is from a one sided level of experience.
Downsides I suppose, are more bulk, and width, making it harder to negociate some cycle path gates and so on and more bulky to store.
Also, I'm not sure about such a young baby in a child seat - I think it all depends on if they can hold their head up. I assume you've checked that, if you're looking at it as an option.