DC1 will be 4 next week and is rear-facing (25kg seat)
DC2 is 6 months and obviously rear-facing, when she outgrows the infant seat we will use the 18kg seat we have (cybex Sirona, it rotates and can be used either way but we only use it rear-facing)
I will put DC1 in a forward-facing HBB when he outgrows his 25kg seat or when DC2 outgrows the 18kg seat, whichever happens first, but I don't think that will happen for a while!
If we travel by plane and hire a car at the other end, I might put DC1 in a HBB, just because it'll be a pain to take two big car seats with us, but it depends if I can find a compact/light HBB with decent safety ratings.