It depends on what role you want it to perform, it seems the Harpy is better at bombing infantry and shooting tanks with the venom cannon. Crone is meant to make sure skies are clear from enemy fliers.
On the reverse side, the Harpy can provide anti flier roles with V-Cannon but not as good as the Crone with Tenticlids+Vector Strike. The Crone can perform anti infantry roles with the drool cannon, but it has to get close to the enemy and its not very durable, so it might be better to use it against isolated scout type units. Also Crone can still kill tanks/MEQ with S8 vector strike.. if there's no fliers enemy fliers on the board I'd just fire the tenticlids at the tanks, because why the hell not
Downside is that Crone has only 4 tenticlids while the Harpy has unlimited spore bombs.
It really depends on what you're trying to do brah