Take your seats with you when you vehicle shop. It's the best way to actually verify that you can fit the seats in terms of width *and* actually reach between the seats to buckle the booster. Also, make sure you do a mock buckling of the Nautilus too.. as down the road when you use it in booster mode you will have to reach between it and whatever is next to it.
I cannot recall if the Tuscon and Equinox have middle head restraints (headrests) in the back. Given that eventually your 10y/o will be riding in just a seatbelt you need to make sure that she has a headrest behind her head, and that wherever she is sitting has a lap/shoulder belt (not a lap-only belt). I don't think either vehicle offers just a lap-only belt in the back, but it's something you need to think about and check on as you shop for this future vehicle.
If whatever you buy does not offer a middle headrest your oldest child will need to sit on the side (outboard). With that in mind, you will have a Scenera and Nautilus right next to eachother... which takes me back to my original point: you need to make sure you can buckle the Nautilus when in booster mode down the road.
Doing a safe and comfortable 3-across is very doable these days, but it often takes some extra forethought and planning. :thumbsup:
HTH's