I'm new to Panerai, so don't hesitate to discount my opinion! To my eye, the 587 is simple, classic, and understated ... if that's possible on a 47mm watch! Plus, there's something about those two words, 'Marina Militare'
The 687 is certainly more eye-catching, with its 'sunburst' dial and striking bezel. For me, the 587 is striking enough with its large and rugged, yet refined, look.
Let us know what you decide!