Which is safer?

[stayinhomewithmy4]

I'm going to help my friend/neighbor with her car seats tonight and I have some questions. She has 4 kids - DS just turned 7, DS almost 5, DS just turned 3, and DD 18 mos. She has a Scenera for the 18 mos old and I'm going to try to convince her to let me turn her around (and I'm hoping the 34" height guideline is on one of the seat's stickers), and she has booster seats for her 3 boys. The one I'm most concerned about is the 3 yr old (turned 3 in May). I would guess that he's around 35 lbs. I have seen the seats in her van and I know that she has low back boosters for the older 2 and a Cosco SightSeer for the 3 yr old (you can see a picture in the pictorial: http://www.carseat.org/Pictorial/7-BPBooster-np.pdf). The seat that he used to be in was an OHS seat that I'm guessing she's had since her 7 yr old was a baby. I had told her about expiration dates a few months ago, so it's possible that she got rid of that seat and that's why she bought the Scenera for her DD, but I just don't feel comfortable with the 3 yr old in a booster seat and I'm worried that she doesn't have anything else for him to use. If she still has the OHS, would that be better than the booster? Definitely not if it's expired, though, I suppose. I do have a Touriva that she could borrow until she gets him a different seat, so that's probably what I'll end up doing, assuming he fits in it. And if he doesn't fit in it, I guess she'll have to borrow my Radian.
I'm really rambling, I know. I'm just concerned about this situation tonight and what is going to happen.
So, here is my question: If the OHS seat is not expired, which is safer for the 3 yr old - the OHS or the booster seat? I'm thinking the OHS, used correctly, but I wanted to get other opinions. TIA!

[papooses]

Unexpired OHS would be safer than a backless booster for the 3 year old -- if it's expired, I'd probably just splurge on a new Scenera for her rather than let her borrow your Radian....

[stayinhomewithmy4]

Unexpired OHS would be safer than a backless booster for the 3 year old -- if it's expired, I'd probably just splurge on a new Scenera for her rather than let her borrow your Radian....
I would love to, especially with the $35 price at Kmart right now. Unfortunately, we're about a 45 minute drive from our nearest Walmart/Kmart/Target stores. They're leaving on a long car trip, driving from NW WI to N Carolina, so I would only be letting her borrow the Radian for that trip and then she'd have to get something else. If he'd fit in the Scenera, though, he'd fit in my Touriva, right? She can have that seat. I'd gladly give it to her and buy a new Scenera for myself.

[papooses]

Probably, yeah -- I think the Touriva has slightly higher top slots than the Scenera? :)