I don't think your DS will need another highchair at that age. We got the booster seat thing as our DD wasn't high enough at the table which made it hard to eat.

There are other brands out there that do a similar seat to the maxi-rider but off hand I don't know who, and I don't know what type of material they are. Maybe look into a booster seat with a H harness like Infa Secure or something. The H harness will be a lot safer than normal seat belt and your little darling shouldn't be able to wiggle out of it either.

As for the Phil & Teds. I never liked the underneath seat either, but the thing that sold me was, the toddler seat sits on top when you have a newborn and its still like sitting in a normal pram so sleeping was never a problem. I got a coccoon when I bought the pram too so that it was easier to get DS in and out when you have the toddler seat on the top.
We hardly use the toddler seat now as DD is 2.5 yrs and walks most places and has done since about 2. If we are out and she does want to sit in the pram I just put her in the bottom seat. She seems happy enough.

Hope that helps. Happy hunting!!