Well, I've owned my '07 Nissan Altima 2.5S since it was new, and have over 53k miles on it.
Only parts replaced have been front brake pads and rotors, plus tires.
Mine has the 6-speed manual transmission, and Nissan no longer offers the Altima in a manual in the US anymore, only Canada gets the manual in North America.
The Sonata is offered in a manual, but only on the most basic version of the car.
You can't even get the nicer looking alloy wheels if you want to have a manual, which is a shame.
I test drove the Sonata back in December, and was really impressed by the car.
I was even impressed by it's automatic transmission, and that wasn't expected.
The DFI engine provided good power, but is a very new technology, and has some potential long term issues, so that's my only concern with what appears to be a very nice car.
If I had to chose between the Altima and the Sonata today to replace my car, I would choose the Sonata since I can get it in a manual transmission, and it has the DFI engine, which produces more power than Altima's comparable 4 cylinder engine.
If you are considering the V-6 Altima vs the Turbo Sonata, I would actually tell you to see if you can get Infiniti to sell you a G25 sedan for a comparable price. I bet you would get it.