Ok. I'd seriously go with the 300 for fleet service, seeing as the Panther platform has been cancelled. Every day Chrysler is winning bids for fleet vehicle service where the old Ford platform used to dominate. I guarantee you that the new Charger/Magnum is what almost every police department in the US will be using within five to ten years. The Texas DPS is already fielding several Charger R/T's as interceptors, as well as a handful of V6's in my area. The V8 Charger flat out demolished the Crown vic at the Michigan State Police and California Highway Patrol testing sessions, whereas the V6 Charger kept up and beat the Crown Vic in many areas.
the 3.5L V6 in the 300 Limited would be a pretty nice choice, seeing as it gets like 19/27 EPA mpg. That engine has 250 hp, 250 lb/ft of torque, so it is no slouch compared to the 4.6L. The 200 hp V6 that was mentioned earlier is the Chrysler 2.7L, a great engine in its own right, but it is so often viewed as underpowered due to the heavy vehicles it is almost constantly installed in. Put that engine in a PT cruiser or Caliber, and it'd fly!
Overall, I'd recommend the Chrysler over the Town Car. Did I mention mercedes-Benz helped design some parts for that 3.5L engine a few years back and the parts for it have been around for a decade? Yeah that's right, you can find that same 3.5L in the old 300M and a few other cars IIRC. Go to Allpar.com if you'd like to know more about any of Chrysler's products. Current and Former DCX engineers regularly frequent and contribute information over there, so you can take the majority of it as gospel.
