New CPU is fast and consume less power

If you buy a new one, I would go with the latest version.

If you already have Core 2 Due, then it's not worth to upgrade to i3/i5.

I do the upgrade for every 2-3 generation. These days new computer(laptop, desktop) is cheap. It's not big to upgrade. After selling the old one, I usually spend less than $300 for the upgrade every 2 years. Plus new laptop LCD panel is brighter.