Road Rippers vs Imaginext Rescue Center

ETA: I vote for getting the Imaginext.
A set/castle thing/structure, etc. My son creates whole cities with it and with all the other toys/parts/figures of stuff he already has. Your son seems to be like that too.
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You already have "a few" road ripper vehicles. So I would not get that. You already have some.
My son, liked Imaginext things at that age. He is 7 now, and occasionally will still play with it.

The thing is, even if something is a "building collection" will your son like it and, build stuff?
All of this is imaginary play. And even if a kid has a "set" of something, mostly, the kid will play as they tend to play.
It not always being the way the toy is meant, to be played.

When my son was 4, he was grown out of the "megablocks" stuff. He was, playing with Legos. The regular "big kid" Legos. His sister's and then we got him his own. And he built his own stuff with it. And was pretty independent about it. He just liked to construct, stuff. And he will OFTEN use stuff from other genre of toys, to construct his own city or scenario or community. So, kids do not just use a toy/brand, with itself, only. They use pieces and parts of other sets, to then combine EVERYTHING and make up their own, thing. My son does that.

If you get the Imaginext stuff, I would suggest getting a set. Meaning, not just the figures or a vehicle. Get the "castle" or "fire station" etc. A set. The whole play set. My son routinely still plays with his Imaginext "castle" that he got once as a set... BUT also uses that castle alone, to then incorporate his OTHER toys/figures/Transformers/Hot Wheels/Marvel Super Heroes, into it too. And other building blocks etc., and combines it all, and makes his own "world."
That is part of the fun.
So, since your son already had Road Ripper stuff, and not the Imaginext... maybe get him the Imaginext.

Have been buying Imaginext since my son was a toddler. He will be 8 in December and STILL plays with his Imagnext sets. They are like a dollhouse for a boy! The toys are good quality and follow themes which makes it easy to add to the sets. We have the jungle, Batman, and Space sets. Legos end up scattered and are easy to lose. Once you misplace a few pieces the set cannot be rebuilt (and they're expensive) My son gets lego sets, I build them following the 325 pages of instructions, he plays w/ it for a few days, then they are disassembled and dumped into the big box of legos! My daughter also plays w/ the Imginext sets (she will be 4 this month)

The rescue center is great! But, I rec buying the helicopter set and some extra imaginext guys to go with it. We play with Batman and other imaginext guys with it. It only comes with 2 guys in the set. Well worth the price. We also LOVE imaginext eagle castle. Imaginext toys are really great. Our road rippers sit in the toy bin.

I've not heard of Road Rippers, so I can't speak to that, but my kids LOOOOOVE their Imaginext toys and they are incredibly durable. Of all the toys they have they get played with the most and are the best value.