I know I am very late with this reply but if you want an Impala then get either an LTZ or SS trim. The rental is probably an LS or LT and these are not the same car as the LTZ or SS.
The other trims seem to have various mechanical problems and I have seen more than a few 2008 LS and LT's being towed away.
As for the Nissan Altima, I raced someone with the 3.5 sport last summer and it was no contest. They are nice looking cars but Asian cars are like cell phones; after two years nobody wants them anymore and you have to upgrade.