You can always check out
www.edmunds.com or
http://www.autotrader.com and for a pretty fair pricing comparison, therre is always
The Kelly Blue Book
My preference between the engines would have to be the 2.7, simply because it is larger, and (I believe) a newer model. I have seen ads for a super charger for a 4 cyl, but I couldn't verify which engine it would fit.