Gen 5 all the way. If you can, look and get a J-VIN.
I own both Generations, so i have no biased opinion.
But i like and prefer my Gen 5, it is a beast, i do all reg maintenance and never had any major problems. I am at 140k miles i think when you convert. And it is still running strong, and i beat it up on the track.
The Gen 6 (XLE V6) Is a nice long distance cruiser, but i don't like the styling of the Gen 6. Not to mention all of the problems with the years 07-09. Mine is a 2010, no major issues. But i still prefer my Gen 5