It truly does not matter if you use an infant seat or a convertible from birth. What DOES matter, however, is how the convertible seat fits an infant. MANY do not fit infants from birth. Even though the weight limits may start at 5lbs, the bottom harness slot needs to be at or BELOW the shoulders for rear facing, and most newborns are too small to use most convertibles from birth because that bottom harness slot is too high. A couple that work reall ywell from birth are the
Cosco Scenera ($50, rear facing 35lbs, forward 40lbs, basic seat, not lots of padding, but works for newborns)
Graco MyRide65 ($150 rear facing 40lbs, forward facing 65lbs. LOTS of padding and infant inserts, great for extended rear facing, great seat)
Those are pretty much the only two who fit newborns from birth well, where the bottom harness slot is level with their shoulders so its acceptable.
However, you may want the convenience of an infant seat at times. We used our sling almost exclusively, however when we went out to eat, th infant seat was irreplacable before baby could sit up properly in a high chair. If you want the most staying power in an infnat seat, look for the Graco SafeSEat 30, or the Chicco Keyfit 30. These will hold children to 30lbs, and most kids can use thes seats until they are 2 (but most parents choose to switch to a rear facingc onvertible around a year old anyway as most kids prefer to sit up more around that time)
Those are the seats I would look for if you want to start with a convertible from birth, or the two infant seats you should consider if you want to use them longer.
We started with an infnat seat with both our boys, and they had grown out of it by 5mos old. We switched to a convertible car seat then and they both rode rear facing to 18mos and 35mos in the convertible.
A few more things for you as a new parent to research is EXTENDED REAR FACING. This is keeping your child rear facing beyond 1yr adn 20lbs. The AAP recommends that children remain rear facing until at least age 2 and 30lbs, or to the limits of the convertible car seat. A childs spine does not ossify (harden) to the skull until a child is 3-6yrs old, so the longer a child stays rear facing, the safer that child is in a crash. Its 5 times safer to a child to be rear facing in a crash after age 1. In Sweden, they rear face to age 5, and they've had fewer deaths in 10yrs than the US has in one day. That speaks VOLUMES to the importance of EXTENDED REAR FACING. Please google it and educate yourself now.
Also I wanted to address the poster who said consider the size of your car- thats not true. If an infant seat fits in the vehicle, a convertible will as well, as they sit more upright,even with a newborn recline, than an infant seat does. I've fit a Britax Marathon, one of the largest convrtible car seats out there, rear facing in a Chevy Aveo... a subcompact, without it evne touching the front seats. You just have to knwo what you are doing, and if you don't, find a CPST at www.safekids.org who can help you.
A car seat is outgrown rear facing when there is less than 1 inch of hard shell above the childs head, or the child goes over the weight limit. Before this happens, the seat is now outgrown, but if even one happens, the seat IS outgrown and needs to be replaced with a larger seat. Its very important that you move the child to a larger seat when there is one inch of shell above the head. That inch is the 'ride up' zone, in a crash wit ha rear facing child you want the seat above the childs head at all times, and that inch above the head ensures that.