The Altima is just a little step high then a Corolla in the fun factor. The Mazda3 beats it by a mile when it comes to handling, oh and gas mileage, the hatchback is very practical, some people may not like the "smiling" grill but it doesn't bother me, and Mazda has a ton of performance parts to get some more HP out of it, if you buy the Mazdaspeed performance parts it will not void your warranty. The Mazda3 is just more youthful and fun then an oversize sedan.
If you know how to drive a stick, look at the Mazdaspeed3, it is just like a Mazda3 hatchback but comes with a factory turbo that brings it to 263 HP and 280 torque, and still gets incredible MPG.
Go to edmunds.com for more info about the cars