Yes, at least in my perspective it is worth buying the older one. I was in a similar position as you, but choosing from the MacBook Pro 13"s Retina vs Non-Retina, and mainly the graphics card. And when I got my intel hd 4000, I was more than satisfied with it. The Iris is better for sure, but the 4000 is amazing as well.
When you it comes down to what you are buying, the QUAD cores, and i7 WITH the Dedicated graphics card GT 650, thats most certainly better than the Iris pro, hands down no doubt.
Only things you will be sacrificing are: The retina, the slimness, the portability (because of the slimness+13 inch).
Things you gain: HDD + RAM upgradability(main reason why I bought the older 13" for myself), optical disk drive.
Now obviously i missed a few things that you'd gain and/or sacrifice but those are the ones that come to my mind right now.
Also, to add on the 2.4ghz vs the 2.6ghz, is not a big factor at all. Those extra 0.2ghz won't do you much good. But the i7, and the QUAD cores will most certainly show you some epic results!
Enjoy! 😀