The 2.5 is the QR25DE engine and the 3.5 is the VQ35DE. While newer versions of the QR have been less problematic, the older versions have had serious manufacturing flaws which have resulting in the entire engine being destroyed. I have a QR powered Sentra SE-R, and it seems I have had to do quite a lot of 'preventative maintenence' to avoid having my engine die. Modding this engine increases the likelyhood of problems down the road in older vesions (up through about 2004). Aftermarket support is decent. It is a torquey engine. The VQ has a good amount of aftermarket support, is a great performer out of the box, and is very, very reliable. It powers the award winning Infiniti G35 (and the 350Z of course), a vehicle Road and Track tested for in excess of 50k miles, and have not needed an ounce of repairs. Hands down, I would reccomend the VQ35DE powered Altima. I would consider the Altima SE-R, as you can get a great dealer markdown, and it has many odds and ends that even the 3.5 SE lacks.SouthernPride said:i was thinking of the 3.5 SE or the 2.5 S. i am going to fix it up any way i go. but i dont know much about them so which one would u guys suggest?