My son is a smaller child and I was able to use his infant seat for the whole first year. I LOVED being able to detach the carseat from the base. But once he was able to sit and ride in shopping carts, I didn't take the infant seat in and out of the car. He still hadn't hit 20 lbs by then, but it was already hard to carry him in the carseat. I just went in and out of the house with it. I wouldn't bother with the seat that goes to a higher weight, b/c you probably can't carry the baby around in it at 30+lbs.
There are some infant seats that are heavier than others too. My sister had a baby 2 mos before me, we both got Gracos and hers is a smaller carseat and stroller than mine. I called mine "The Beast". The stroller was too wide to fit in aisles of stores, too big for car trunks, the carseat was heavier and too big to fit in some smaller spaces, like restaurant booths. Also, my bigger seat didn't fit on a single shopping cart the way you see them on the handle. Whichever kind you get, make sure the seat is secure up there, or it can fall and the baby can crack their skulls. My seat had some complaints of exactly that. Although I am not sure why the mom would be so stupid to put the seat up there when it obviously didn't fit and wasn't secure. My sister's smaller set was much better. I would use the size and weight of the seat and stroller as one of my criteria for picking. Next time I will get a smaller set. I loved having the set, but mine was just too big. I believe it was the Graco Quattro, but I am not sure.
Infant seats are supposed to be safer for infants than convertibles. I know you can put a newborn in a convertible, but I have read that isn't a good idea. Just another reason to get the kind with a base. The base makes it safer too, because you don't have to check that it is properly installed every single time. Definitely get your installation checked by a pro (call your fire dept or police dept, they do them free). And once baby outgrows the infant, get a good convertible that can be used for longer, ours goes up to a pretty high weight and height. I would spend more money on that than on the infant seat since you will use it for years. And keep baby rear-facing as long as possible, at least 1 yr and if possible up to 2. And NEVER buy used, not worth the savings there. Five-point harnesses are safest also, for both infant and convertible (and I guess boosters too, but we aren't there yet). Hope all that helps.