here you go...

1) not in the same class

2) not in the same size

3) cost of maintenance - altima is much lower (when i did diy on jetta, more than often i questioned their design being unnecessary unhuman; sometimes need special tools)

4) altima is more comfortable on the road 

5) altima is quieter - jetta road noise is louder

6) both are unibody.  jetta bottom is just a thin layer of metal sheet.  i wish i could give it a higher score on safty but i can't.

7) jetta handling recalls has been just like toyota - knowing the problems existing but denies and denies until arms are being twisted.  i am not sure what model/year you are buying, in the past, its diesel engines had big flaws and even to date it is yet to recall.   (mine was covered by them eventually otherwise i would have lost 5k on the engine).