Personally, I did not notice any difference in accuracy when I bought my first 455; I have since owned 5, and still have two 455's, both in .17HMR, and they are the most accurate .17's I've ever owned. One is consistently a .6" or .7" gun; the other is consistently a .5" to .6".
I also still own one 452 in .17HM2. It is consistently a 1.1" or so gun.
I don't think the difference in the actions has nearly as much bearing on accuracy as the individual barrel. I wouldn't really sweat the whole 452 vs 455 thing.
If I was in your shoes, and the irons were important to me, I would buy the 455 Trainer. If a 455 American is available, and the iron sights aren't a big deal, I would take 455 American. The trainer stock is OK for a scope - at least for my face; the American is better with the scope.
Having the ability to switch calibers is a plus, and I did that for a little while, but then just found myself buying more rifles....Where it is nice is changing, or upgrading barrels is not a gunsmith job - ala throwing a Lilja on.