well, you are going to move baby into a convertible regardless, so my thought is its not such a big deal how long the infant seat will last. if you want the ability to use as a travel system, i would go for the lightest possible infant seat. its not like using the infant seat longer will allow you to skip buying the convertible, ykwim?
and fwiw, I skipped the infant seat with my last 3 babies and didn't miss it. i used a radian and a cocorro from birth and both are great. I also love having a stroller with full recline and/or bassinet. especially with older siblings, activities, etc, the bassinet was wonderful. dd2 would sleep and sleep and sleep in it. i also used it at home for naps when she was tiny.
loving my dh and our littles (dd ~ 11 yrs, ds ~ 9 yrs, ds ~ 7 yrs, dd ~ 5 yrs and baby brother ~ 20 mo)