Me, personally, I would buy mega-head protection (given what's out there, which of course isn't perfect) no matter what. Accidents freak me out!
I am grateful for side air-bags (and regret we don't have them in one car), but I don't want to rely on that alone and not do everything possible to protect the kids. And, it doesn't seem that mega head-protection need be cost prohibitive. The My-Ride looks very deep for an older toddler, for example. I'm the crazy momma who was insistent upon lots of side-impact protection and head protection for my child in the middle row, middle seat of our van with side-curtain air-bags!
It's possible the carseat on either side of her would jam into her anyway, so I go all out.
But also, I am always asking why the carseat makers don't jut the head protection out a lot more. Would it really be that hard? We know the forward motion of a crash will push kids out of position anyway, so why not go way overboard with head protection? But that's just me. Seems the Prosport is my favorite for our Civic without airbags.