The 7/17 shimmershell is indeed the good one. Absorb lets you convert power directly into health, in addition to doing more damage to the enemy.
For the Rapana Whelk, the breed 3/13 isn't quite as good as the breed 12, but the difference isn't major. I leveled my breed 3, he's perfectly good but has been replaced by the shimmershell 7. The breed 5 is pretty far behind any of these options.