There is no big price difference between the two *right now*. Just go with GD04, that's better cosmetically. If the op found GD05 for $30-$40 cheaper and bought it, then that's the point he has to compromise with.
A good solution, except that generally a slim BD drive is slower than a normal one in reading BD-ROM (when ripping BD). FP58 accepts only a slot-loading drive. Among these drives, the fastest are Sony-Optiarc BC-5650H and BD-5850H, BD-ROM *DL (Dual-Layer)* read x6, but they are expensive, ~$120 or more. LG CA10N is slower (x4.8) and cheaper (~$80). Many older, cheaper drives are only read x2.
BTW a normal BD drive of BD-ROM DL read x8 can be had for $70.