Which is safer: backless booster in center or HBB outboard?

[Aishe]

DD (6yo) rides in a Frontier in my car, which she rides in about 99% of the time. She has/had a backless booster in DH's car, but I preferred a HBB for her, especially since she occasionally rides in my brother's car, which has no side airbags. So I just bought the new Turbobooster that Jools posted about (in the green, which I love!). It doesn't fit in the center seat next to DS's BLVD, so I put it outboard. Which made me wonder: which is safer, the backless in the center or the HBB outboard? Or is it six of one, half a dozen of the other? DH's car is a Mazda 6 with side airbags, if it makes a difference.

[Joolsplus2]

Gotta flip a coin, we really don't know. One of those questions I struggle with constantly, and then I just have to go with what's most convenient and comfy (though, I find my own 6 yo dd seems to have such a tiny shoulder, it's only seconds of partial leaning before she's way out of the belt in her backless, we've gone to a highback outboard for now ...)

[Aishe]

Thanks, Jools, that's kind of what I figured. She really likes the Turbobooster and says it's a lot more comfortable than her Harmony Cruz so I think we'll stick with it for now. Thank you for posting about the new Turbobooster!