I have driven (and owned a couple) Police 5.7 Chargers, and owned one SRT8.

I would definitely pick stay with '09 and up 5.7 because of the change to Eagle Heads and VVT with the jump in HP. Although the earlier ones were still very fast Police cars here, and we didn't experience a bunch of engine trouble, they had that reputation.

I don't know what it is, but I swear, there is something about the '09 and '10 5.7's in the Chargers (at least the Police Chargers). They just flat out plant you back in your seat! And they sound better than the later ones too. I can't explain why, but I have driven enough and heard enough to say there is something with the '09 and '10. And others have mentioned the same thing to me. If you can grab one I'd highly recommend. My '06 SRT8 was a great car to me, hauled butt and was trouble free from 28,000 miles. I was not a lead foot except for the occasional necessary stomping it onto the freeway. I averaged 18mpg. That's what I'm averaging in my 5.7 with MDS.

I don't know anything about the 300's.