Where do you get these "makes no difference" assumptions from? They're certainly not based in reality, that's for sure.
The difference is not just 0.1 Ghz. The difference is in the CPU cache. 3 Mb on the 2.4 vs. 6 MB on the 2.5.
That's a *huge* difference, particularly when it comes to tasks like photo editing. It will be immediately noticeable with the very image you edit in Photoshop. Anyone who says otherwise has never actually compared the two (i.e. they're full of BS) and has no grasp on the relationship between hardware and performance.
Don't believe me? Go google up some benchmarks - doesn't even have to be laptop benchmarks, PC benchmarks will prove the point. Now find the numbers for a 3 MB CPU and compare them to a 6 MB CPU of similar clock speed. The difference is not slight.
Personally, I wouldn't even consider the 2.4. It's a waste of money when for $500 more, you can double your CPU cache (on top of the other upgraded bits). If you're set on the 2.4, you might as well save a few bucks and drop down to a regular Macbook 2.4 - it's the exact same CPU!! IMO, the only reason to get the MBP over the Macbook is for the 6 MB CPU (and discreet gfx, if you need that).