car seat options

I still have my daughters infant car seat but she's going to be almost 4 when I deliver our baby so she is very helpful and can do a lot for herself, what do you think about getting a convertible car seat for the new baby and using a ergo or similar and a stroller instead of having to lug around the heavy baby carrier? Have any of you found that to be difficult and it easier to just have the infant car seat? I'm a stay at home mom and don't leave the house all too often.