We went to a Car Seat Safety Class today, taught by a car seat tech with national certification, etc. Thought I'd share what I learned!
Bucket infant seat vs. convertible seat - both are equally effective as long as they're rear-facing. The convertible seat becomes less safe once it's turned around. She said that the bucket seat is great for portability, and as part of a travel system, but a RF convertible seat is absolutely fine for a newborn, as long as it's rated for that weight. (I was totally surprised). The RF convertible carseats have the ability to recline, so that's the necessary part for a newborn, to make sure the head is tilted back with ample angle for an open airway. She said most if not all have an insert for small infants, but if for some bizarre reason it doesn't, you can used rolled receiving blankets on either side of the baby's head for additional support.
Weight limit of infant bucket seats - 22lb, 30lb, or 35lb really doesn't matter. None is more safe than the other. It's about what's convenient for the parents. She said that she can't imagine anyone wanting to tote around a 35lb infant in a bucket seat, but if you just want to be able to postpone buying the convertible seat for longer that's fine.
Rear-facing length - as long as the kid can stand it, minimum of two years. We watched these awful (yet informative) accident videos, and as horrid as they were, they proved the point that a 20lb infant does not have the muscle tone to handle a crash when they are forward-facing. Their feet can touch the back of the back seat, it causes no problems (again a surprise).
Safety-rating - if it's on the market, it meets minimum safety standards, which is front and rear impact crashes. They do not routinely test side impact. If a manufacturer states that it's better for side-crashes, there's no way to verify that really. If money is an issue, it's more important to have a seat installed properly and used consistently and correctly than to pay for one that independently claims better safety. She said that you can call the fire department or police department and they usually have a certified technician who can check your installation and usage, as well as perform periodic checks to see if the seat or base is getting stress marks, or starting to weaken in any way. She reiterated not getting second-hand car seats unless you personally know the seller.
Other bits - Babies should not have on thick layers of clothing (jackets and sweaters) on when strapped in the car seat, as it creates too much slack in the harness. Aftermarket car seat additions (padding for straps, cushier head inserts) are okay as long as they do not alter the function of the harness; some strap pads are so long they cause the harness clip to slide down to the infants waist. They need to be short so the clip holding the right and left strap together is armpit level across the breastbone.
We thought the class was pretty great. Hope that helps!