It really boils down to what you want. Both are good cars and should last a long time. The Altima is a considerably bigger car than the Sentra. So if you plan on hauling people/stuff, the Altima is more practical/comfortable despite the slightly lower gas mileage. But if you spend a lotta time in the city and parallel park, the smaller Sentra might be a better choice. I had an Altima SL sedan as my wife's DD for many years and it went on many long road trips without any fuss. It was far from a canyon carver, but it never stranded us and returned good gas mileage. A solid car for many years. My wife's only complaint was it had a bigger than normal turning radius, which simply meant a slight inconvenience in tight parking lot maneuvers. As far as newer Alti's, you'll hear a lotta hype about the "NASA inspired zero gravity" seats which is advertising fancy speak for memory foam.

But once your tush sinks into the seat, they are actually pretty comfortable. Good luck.