If you can afford it, safety experts recommend buying an infant only carseat because it fits little babies better than a convertible.
As for the stroller/carseat combo, I wouldn't buy one. Get the infant carrier carseat you want, then one of the bases made to hold it. (Ie, the metal and plastic frame that is useless without the car seat in it but is great for holding and transporting the car seat.) It's light and easy to use, and it's relatively inexpensive. In my experience, the stroller that looks good when you're planning for a newborn is generally not what you'd choose 6 or 9 months later when you're buying for a mobile baby who can sit up and won't always be sleeping in it. I'd buy the frame at first, then get a real stroller later on when they know more about what they want.
As for the infant seat size, I had the smaller one for my first 2 kids, and they outgrew it by 6 months. I bought the bigger size for my 3rd since the first car seat was too old, and I liked it so much more. It was heavier so I was more likely to use it with the frame than hand carry it, but it was so nice to be able to take it in and out of the car when I had to drop an older child off at school or run in someplace when the baby was sleeping. Plus it was nice to be able to use it instead of a high chair at people's houses. DD was in it until about 15 months in the car, but I probably only took it out until 10 or 12 months. Loved not having to give up the portability at 6 months, when mine were still falling asleep in the car frequently.