I will go for the 2.8GHz. It is a 16% performance increase over the 2.4GHz, something that I believe you will notice.
The processor is not something you are likely to upgrade in an iMac, and OSX is a hungry servant... I think the $250 are well spend. I don't want to spend the next two years thinking what might have been for $250 more.
If you want to minimize cost, you can chose the 2.8GHz BTO upgrade on the 2.4GHz version, and then buy the last GB of RAM from a third-party vendor. It is the same RAM-spec as the old iMac. That is doable for the $150 price-cut, with money left to spend.
I think of it this way: I was ready to spend a lot more on the old 2.4GHz, and get much less. In that light the 2.8 looks pretty-darn-cheap.
That is my take on the story...