which truck to buy & what motor should i b looking for

Kenworth is a Lincoln Towncar and Freightliner is a Plymouth Breeze. That ought to narrow it down.

 

From the options you have stated, i take it that you want a traditional style rig. KW and peterbilt are built by PACCAR and use many of the same parts depending on the model. They are pretty much deal even in quality. For the aeros, the KW T2000 and the T700 are the twins to the Pete 387 and 587. The only difference between the two is the dash, hood and front clip. The rest is identical.
If you want a traditional rid go with the KW W900L or the Pete 379/389.
If your serious about saving some money and driving OTR go with the aero cabs. From what i can tell they will save a few grand a year in fuel costs.
The freighliners do not have a real good fit/finish to them as they are mostly intended as fleet vehicles where weight / cost is most important.