You need to work out what is best for your particular usage patterns.
If you are a heavy gamer, then subject to increasing the RAM, you may find the 2.4GHz plus 9600 better than the 2.53+9400.
If however battery life is a concern then you would want to run the 2.4GHz with its 9400M anyway, in which case you're definitely losing some performance (though not much) and you wouldn't get close to the battery life the newer model would give you. I also don't know off the top of my head whether there's any difference in the CPU cache between the two models.
IF battery life is a concern then again you have a choice to make. Out of the box, the new model with non-removable battery has substantially longer battery life (~7-8 hours instead of ~5, I believe, but I am only going on the posts on here - I've not tried the new models). On the other hand, the older model has shorter battery life but you can carry a spare battery, thus its maximum battery life is limited only by what you want to carry.
If you're not someone who would be likely to want more than 7 hours, and are not someone who would be likely to buy and carry second batteries, then the newer one would likely be the better option.
If you do keep it, I would try to politely insist they at least give you either a 4GB upgrade for free or a decent discount to reflect that. 4GB makes quite a difference to performance with multitasking if you run a lot of applications, or do image-heavy work. If you did decide to keep it and wanted to upgrade the RAM yourself then crucial.com is usually a good place to look (am I allowed to say that here? I'm not affiliated...).
The one you have also has an expresscard slot, whereas the new model doesn't. Again, the value of this depends on you as a user. There's no doubt the expresscard is far more flexible - you can add an SD reader if you want, but it can also add things like e-sata or compact flash readers etc. It may also help its resale value to certain buyers, as currently the only way to get an expresscard is to buy a 17" MBP. However if all you ever wanted was an SD card reader, you may not care about the Expresscard. I don't think Expresscard can be a *disadvantage* but it may not be an advantage *to you*, in which case consider the other features.
You also have a faster SATA interface. This will make no difference with an ordinary hard drive as far as anyone can work out, if you don't have or plan to buy an SSD drive to replace the internal harddrive, but if you did, then unless Apple fix the furore (see other threads) over the new ones having slower SATA then yours would be quicker (though again, chances are it would still be very quick either way).
Sorry this is all very wordy, but what it all really boils down to is that it depends on what you see yourself wanting out of the machine. If you are someone who buys a machine and uses it as it comes, doesn't tend to upgrade/buy accessories etc and will just buy another machine one day, then the newer model may well be better - there's no doubt its battery life (on the built in battery) would be longer, but if you are after flexibility then the older one that you have offers more.
If you *do* keep it, I'd try and get some further discount (or Applecare or something) though!
I suspect for most 'ordinary' users (whatever that means!) the newer model would be more useful, but only you can work out what kind of user you are. Best of luck. Hope you're happy with whatever you choose.
. It may well have an inferior drive as well, allthough it'll run SSD better, as a myriad of posts will tell you elsewhere on this forum.
