spare carseat/booster

[lcl]

i'm having a diffficult time figuring out what the next carseat/booster to get. we need a spare one for grandpa and /or nanny. we have two marathons already (one for my car and one for my husbands). my son is 3 yo and 40" and 29 lb. we are expecting our second child soon.

we also have a honda pilot. should we get a costco touriva/scenera or an evenflo generations or graco ultracargo? Thanks