Ah, rewritable DVD combination drives--what a mess. Drive-makers need to settle on a single recordable standard, and soon. But of course, they won't, which, ironically, removes the problem of product obsolescence from this category. You'll be buying your second rewritable DVD by the time the market winner emerges, and it will probably be much cheaper, faster and better.
What the diverse recording standards will do, however, is force you to choose the drive features that best fit your primary application--DVD-RAM for random access data storage and backup, DVD-RW for training film mastering with version control, and DVD+RW for mastering with wider reader compatibility. Buy one or more portable drives for use with different desktops.
A DVD-ROM/CD-RW combination drive is a no-brainer, and if you've bought a desktop lately, one of these was probably built into it. This gives you the ability to burn rewritable CD-ROMs and play DVD-ROM and write-once DVD-R discs.