If you're comparing the I5-2400 vs the I5-2500... I'd say there's not much difference so just go w/ the cheaper one.
But if you're comparing the I5-2400 vs the I5-2500K then I would say that you have to consider that the 2500k is easily to overclock (using the built in utilities on most of the current motherboards even) so the difference between them starts to grow.
Personally I think it's worth the 25-30 price difference to get the 2500k.
From a raw processing standpoint the Core i5-2500K offers very little over the cheaper Core i5-2400. It does hold three distinctions, however: it's clocked a few hundred MHz higher, it comes with Intel HD 3000 graphics, and it has an unlocked CPU multiplier.
The 200 MHz (300 MHz with Turbo Boost) advantage is almost insignificant, and gamers with discrete graphics cards will care little about the integrated graphics engine. But the unlocked CPU multiplier is a must for overclockers using any Sandy Bridge-based CPU. The Core i5-2500K is the obvious choice for gamers looking for the best combination of brute gaming force and tweakability.
If you're not going to overclock then the 2500 starts to look very much like the 2400... which in turn looks very much like the 2300. They have the price points calculated precisely to make sure that almost everyone finds something in their budget.
You just need to decide what your budget is, and what option gives you the most bang for your buck.
We can't really answer any better than that without having more information on your budget and what you're trying to accomplish.
You can overclock all of them, but the 'K' series processors have an unlocked multiplier that gives you a greater degree of control with less extreme voltage. So in the end a 'K' chip would be able to achieve a higher overclock rate with greater stability and less risk to component lifespan.
Are you looking for secod generation? And are you looing for a quad core of a dual core? I think the best choice for the first gen, since you do not like the second. The best one would be Intel core i5 760. But make sure to take a second look into the second gens.