More than equipment is involved in safety!
I have worked as a bike mechanic for over ten years now and I must say that the bike seats they have today are much safer than they ever were. However, it takes more than a good bike seat/trailer to ensure safety.
Before you buy:
#1 Choose between trailer and bike seat. Trailers are much more expensive, but do offer some measure of comfort because the baby will be closer to the ground, however, many trailers do not have good seat positioning or view so the baby might have to lean forward to look out. As cute as the rider's butt/rear wheel may seem, it gets boring after awhile. Several trailers now have a safety coupling so if the bike falls over, the trailer won't.
Trailers require a bit more room on the road, so you must think also about where you are going to use it. Bike trails seem safe, but remember, that is where a lot of inexperienced and therefore dangerous riders go. You must consider there needs as well as your own. If you live in an area that has safe wide two lane trails or good wide road shoulders then the trailer might be the thing. Personally, I have a hard time justifying the cost of the trailer because I don't see myself getting much use out of it where we live, certainly won't use it much after dd is grown.
Trailers do have higher weight capacities but I wonder if the child at those higher weights will enjoy being towed around as much. Bike seat weight capacites are approx 45lbs., some trailers up to 75lbs.
Attachable child seats offer a few advantages over the trailers besides price IMHO: Child will have a really good view of what's going on around them.
Child will be closer to you so when a crisis (ie., crying fit or sceaming)
occurs you can speak to them easily.
Many models offer a quick release mount to a rack which stays
mounted on the bike, so when dd or ds has outgrown the seat you still
have something useful left over.
And of course there are disadvantages:
Extra weight on the rear of the bike affects handling, increases rear tire
Seats require a little more work to install, esp. if you do not have a late
model bike with rack fittings on the back. Most hybrids and mountain
bikes have them, some higher end bikes don't.
The fear factor: Child is higher off of the ground. If bike tips over the
seat will as well.
#2 Get it at a bike shop! Install it Correctly!
Even if you are technically saavy, realize that most shops out of safety concerns will freely install bike seats, or charge very little to do so. Also, many times modifications must be made to appropriately set up the seat, shops will have tons of brackets and bolts on hand to do this. If something on the seat is missing, lost or tossed away you can always get an exact replacement from the shop.
I have seen wonderful bike seats with one or more bolts loose or missing completely unbeknownst the owner. They were quite lucky nohing happened. Point is, if it isn't installed properly then it will never be safe.
#3 The shop I've worked at carries only one child seat carrier, by Topeak. It has a super strong mounting system with convient seat quick release, high sides to protect child, adjustable foot straps and removeable washable cover. It is the only baby seat installtion I don't cring in reaction to.
#4 If you do get a seat carrier then do one of two things:
Never leave child in the seat alone with the kickstand holding up the bike!
Get rid of the kickstand on your bike and always ride with someone else so they can hold the bike while you put baby in or take her out of the seat.
The child's weight inevitable wriggling will tip the bike over. The kick stand can only safely hold about 50 pounds total.
I am going to save this reply on my computer because you are not the first intrepid cycling family to have posed these questions, and I am always glad to help out anyone with cycling related questions.
Good luck and have a happy safe time!