I have both and one of my 'feeders' acts like he hates me (I refuse to believe this, he's obviously just afraid because he's in love with me) the other one is bonding with me through his illness, but I'd say they're just more happy to have a big cage, free time, toys and good food every day than anything. One of my boys came from a rescue and the other was bred there where they had big cages and lots of good food and play time, and they act like spoiled brats.
If they're at all grateful it's probably just because they came from a bad place and feel better. And I had two boys that were children of feeder rats and had the absolute best temperaments and were completely loving but had the worst bodies. Not healthy at all.
And two breeder rats. One didn't like people at all, but wasn't aggressive he just never opened up and the other was my first
rat, he gave me a nice big scar on my finger the day we met.
But yeah, I'd say it's mostly background if they're happier but I'd expect more of them to be frightened or troubled unless you get them at babies. They're all so different though. Like I said one of my breeder rats who came from a very loving place just never really cared for people.
My current breeder boy is the smartest rat I've ever had. But I read recently in a rat book that rats who grow up with multiple rats grow larger brains, so maybe they are, but I think all rats are smart.