Hmmm, it is a short seat, but Britax is the only one with the actual SG clip for anti-submarining. There is a lot of debate on whether or not to booster under 40lbs. I'd probably hold off with a new 5 year old that is that small. Maybe you could get a nautilus or other combo seat that she really likes and can use as a booster later on. If she's that small, she may not fit shoulder belts well either.
Yeah I've been over the booster vs. harness debate in my head for months, and ultimately I'm more comfortable with my mom using a booster with her. My mom has had several issues using the harness (not tightening the straps, not moving up the chest clip, not top tethering properly, not locking the seatbelt for the install). She's much better at using a booster because she's been using one for my nephew for several years. I'd rather DD be in a properly fitting boster than a misused harness seat.
I plan to train DD for a bit so I can see that she's got the hang of a booster (maturity wise) and knows how to buckle herself in properly. That way all Grandma has to do is re-check what DD has done and there's not a huge install process like with the Radian she's been using.
I'm also not comfortable using a heavy combination seat in booster mode in their van without LATCH, so I don't want to go that route.
I worry she might be too small for all boosters, and if that's the case, then I will definitely have to hold off. She does have a very short torso (like, only 13.5" at last check), so I guess we'll see. I think the Vivo might be our best since it's shallow and kind of short. But I might end up having to return it!