DVD-R/RW supported by Pioneer came first has larger user base & cheaper drives/media.
DVD+R/RW supported by Sony,HP & Philips , technologically more advanced & playing catch up.
DVD-RAM original re-writeable format from Panasonic (limited scope as discs come in cartridges but has advantages of longer lasting, better for archiving & has phase change (can record whilst playing back at the same time))
You could hedge you bets & go for the new Sony DRU-500 which supports DVD-R/RW & DVD+R/RW
i'll probably go with the sony dru500. there's no reason not to, especially if it burns +r and -r. compusa sells it for $340. is there a better deal somewhere?