Hi,
There is one big plus with an infant seat: when the baby falls asleep in it, which she inevitably will, you won't have to take the baby out of the seat to move her -- you can just let her keep sleeping while you lift the seat out of the car or stroller & take it inside. This may not seem like a big deal now, but it's really a lifesaver! This makes the stroller adapter very worthwhile, because you can move from car to stroller or stroller to house easily without disturbing baby.
I found that the infant seat didn't become too heavy until my baby was about 15 lbs, which is a pretty long time. For me though, my baby gained weight so quickly that I had to move her to a convertible seat at 6 months! I got the Radian 80, you might want to check it out. It folds up and has a carrying strap -- great for the airport.
Hope this helps...