FW gives you a speed advantage over USB2, which can be critical during burning. Exchanging internal drives in an iMac is a labor intensive job and hence expensive. If you can do it yourself, you can save a bundle, but it's not for the faint of heart. If you want to sell the iMac in the future, it's time enough to replace the drive then, should it be necessary for a sale. But you can always discount the iMac by the cost of a drive replacement, and save yourself the hassle.
