My homebuilt desktop has an overclocked quad core i7 that would completely demolish just about any mobile CPU in any benchmark or encoding test. My laptop, meanwhile, is a mid-2010 base Macbook with the 2.4 GHz P8600 C2D. Like you, I was attracted to the portability of the 13" form factor, and didn't really intend to use the laptop for serious number crunching (why would I, when I already have a powerful desktop?).
I can't tell the difference between them in typical day to day use (e.g. web browsing, email, documents/productivity, programming, moderate Photoshop and light gaming). Honest. There are a few comparable situations where the desktop seems a bit faster (loading large programs, mostly), but this is almost certainly due to the fact that it has a much faster hard disk setup (a handful of three platter 7200 rpm drives in a RAID array).
For what it's worth, I was still using a Core 2 Duo E6420 (2.13 Ghz desktop model) for
gaming as of this past March.
How much slower is the 2.4 or 2.66 core2duo in comparison to the base i5 2.4? Is it worth the $600 more, and will it last longer,etc?
Probably not worth it, given what you're telling us. You'd notice a bit of a difference (maybe 10-15%) when performing CPU intensive tasks like video encoding or compiling an operating system kernel or something, but I think your intentions preclude such things.
Take some of the $600 and put a solid state drive into your 13" MBP. A computer with a C2D and a SSD will seem significantly faster in day to day use than another machine with an i5 and a 5400 rpm magnetic HDD. The SSD should (theoretically) improve reliability as well, as it has no moving parts--there's no risk of damage from moving the computer while it's reading or writing, for instance.
What's the life expectancy on C2D machines? If they are well taken care of physically and maybe the battery is replaced, should a typical C2D last for three or four more years on a three year old machine???
Any electronic CPU will eventually fail due to a phenomenon known as electromigration (basically, the atomic structure of the copper circuitry inside of a CPU is physically imperfect, and this structure will over time become displaced/damaged due to collisions with the electrons that constitute electric current flow--this is also why your processor produces heat), but this takes a very long time. Intel and AMD have both casually and unofficially suggested that the MTBF for a modern processor should comfortably exceed
ten years of continuous use. Your computer itself will probably not function that long without repairs of some sort, but it'll almost certainly be due to another component failure (probably mechanical--hard drives, fans, etc).