The problem I had with them destroying Defiant and then bringing her right back is it was just a shock moment for the writing staff to throw in there. I suspect they were trying to re-create that effect on the audience as when the Odyssey was dive-bombed by the Jem'Hadar. When the preview for that first appeared in syndication, everyone thought it was the Enterprise.
But obviously they only got a similar class ship so that the production didn't have to build an entirely new bridge set before wrapping up the series.
Yes, I felt exactly the same way. I remember feeling very shocked when the Defiant was destroyed. Then I immediately felt cheated of that shock a few eps later when they got a new one. But that was nothing compared to the irritation I felt when they ended up using stock footage of a previous battle for the finale, which was when I realized that that was why the "new" Defiant was the same as the old one.
I think the only similar instance that irritated me more was when the Delta Flyer was blown into itty-bitty bits in a season finale, only to be shown to be "rebuilt" exactly as before an episode later. That was even more of a cheap cop-out than the Defiant was.
As for the OP, I don't think any ship was more powerful than another, just more technically advanced.