I have the Dodge Magnum (which is essentially the same car as the Charger, only a different body) V6 with the 3.5L. I test drove the 2.7L and it was crap. A very small motor for a very large car. Also, the Dodge 2.7L engine is one of the WORST engines to ever have been produced in america. I took both the 2.7 and the 3.5 to my mechanic to have him check them out before I purchased, and the 2.7 was already failing. He told me to avoid that engine like the plague.
Not to mention the HP difference in the engines is 60 (I think). If I recall correctly, the 2.7 gets 190 and the 3.5 gets 250. Go with the 3.5L, you won't regret it!