Aye, there's no reason to buy a G27 if you can get a G25 cheaper, the G27 is a small improvement in some ways, having a few more buttons and a slightly nicer feel to the FF and shifting, and you can adjust the position of the pedals a bit, it also has an LED rev-counter, but it's also a step backwards in other ways, it's gear wont work in sequential mode anymore (so you can't use it with older/more casual games that don't support a H-shift, you have to use the flappy-padels, whereas on the G25 you had a choice to use the stick aswell in sequential mode), and thouse 6 buttons on the wheel are placed poorly, it's quite easy to hit one by accident when turning, and thouse LED's are only supported by a few games at the moment.
So if you can get the G25 cheaper, then it's good value for your bux, the G27 is not a big enough improvement to warrent the extra cost (if you have to pay extra, anyway), hell, with the G25 getting replaced and all, there's never been a better time to buy one, most places they will go cheap (much cheaper than they are worth), as they are trying to get them out of stock to make room for the G27.