They both have turbo boost. From what I can tell Apple charges $200 more ($150 more for students) for a 153 Mhz boost and 180GB more hard drive space. Ridiculous considering you can EASILY overclock any i5 153Mhz more and a 500GB 7200RPM laptop hardrive can be had in under $80 dollars. Many manufacturer's do this because people don't really know about the sweet spots when it comes to specs. With the i7 upgrade you may actually see a difference, because it has some more cache and more graphics memory, but IMO, still not worth it for the huge price premium.
I am getting the 2.4Ghz in a few weeks. If you really need the hard drive space, buy a 7200RPM 500GB HD from newegg for ~$80 and keep the 320GB hard drive it comes with as a spare or get an external enclosure for it for ~$20. 153Mhz overclocking isn't even worth it but it can be done.
Edit: Just noticed the stock 320GB and 500GB hard drives are 5400RPM, making the upgrade to the 2.53Ghz model even less worth it.