Which Seat for less used vehicle?

Because I am a planner and can't help myself, I am starting to look at the next step seat for my littlest guy once he outgrows his Keyfit (or I get sick of lugging it around). We have 3 vehicles that he will need seats in: a 2006 Freestar minivan (our main vehicle); a 2007 Kia Spectra (daily one-way to daycare about 3 miles and occasional around town); and a 2009ish Saturn Vue (about once a week trips with Grandma).

We have an Evenflo Triumph ProComfort for the van. I love the seat for the harness and tightening mechanism and want to test to see if the size would fit in the Kia. If so, would love another, but I promised DH we were done buying seats for us. We have a Guide 65 that was used for DS1 in the Kia and we were planning to use for DS2. The plan was to buy a Scenera NEXT for Grandma's Vue.

Which brings me to my question: I am a little concerned about side impact protection with my tiny car and having both kids outboard. The Guide seems to have such low sides, I am wondering if the NEXT would be a better fit for a daily driver. Anyone have both seats?

I guess my options are:
1) Buy the NEXT for Grandma's Vue and use the Guide for my Kia (as planned)
2) Buy the NEXT for my Kia and use the Guide in Grandma's Vue
3) Spend a little extra, buy a Triumph and use the Guide in Grandma's Vue

The Guide is bigger until 22 lbs when it can go more upright, a constraint the NEXT doesn't have. The Guide does have head containment wings.

Any rear facing seat provides a whole lot of side impact protection though. I wouldn't worry too much about that.

I have both a NEXT and a Guide. I like the NEXT better (mostly because it's easier to install). I've honestly never thought about the difference in height of the sides.