Both series are very good. While its hard to recommend either series due to the vast difference in the setting/plot/characters I always do, simply because both anime have very high production values.
The story telling is brilliant in both with perfect pacing (although Gungrave's is better). The anime have a great respect for bonds and how characters change. If you like gungrave and dont mind some comedy Trigun is perfect. If you like Trigun and dont mind darker anime's then Gungrave is perfect too.
Lastly the music in both is spot on ;).