Convertible carseats vs infant seats

Has anyone gone with convertible car seats over infant car seats? I'm trying to weigh the options that may be more cost effective to get convertible car seats that can go from infant to toddler insead of buying two infant seats then in about 8 months buying two regular car seats(I say 8months cause that's when my daughter out grew her infant seat) then my question is if you don't have infant car seats how to you put infants in the stroller? Any suggestions appreciated.