Every seat passes the same standards & that fits the child, fits the vehicle & is always used correctly will protect your child. However, seats with energy absorbing foam can help reduce injury risk (although there is no specific data comparison on that). It is most important that you have read & followed the vehicle + carseat manuals. Seats have different ease of use features, so it's important that you feel comfortable with your seat's features.
I would have your friend complete the Used Seat Checklist, signing & dating it >
http://800bucklup.org/parent/used_seat_check.html
Infant-only seats are outgrown so quickly, perhaps we can also begin helping you narrow down your selection of
convertibles to buy after the infant-only seat....
What is the vehicle year/make/model?
Does/will your child ride in a different vehicle? How often?
What is your budget?
Are there other backseat passengers?
What are their ages/weights/heights?
Other safety seats in the vehicle(s)?