From doing a few minutes of Google work & knowledge of Pioneer's general naming scheme, I can give you a few educated guesses:
S07 (BDR-S07XLT) is the premium drive that supports the most hardware features and, as you noticed, sports the nicest look. It also supports a few extra features and can be used with a few dedicated utilities to make the drive quiet or make it perform at maximum performance.
207M (BDR-207MBK) is the next drive in the tier, losing the fancy appearance, but still supports BDXL and all other hardware features. You lose the geeky ability to control the drive noise level and a few other premium features, but that is only a tiny thing, and the drive would still be perfectly capable and is still premium in its own right.
207E (BDR-207EBK): best guess, European version of the 207M. Retains all features of the 207M.
207D (BDR-207DBK): the most basic of the drives. Supports the features that would be most used by 99% of users. Drops DVD-RAM support and BDXL support. Is otherwise still completely feature rich and supports the mot common media available.