I think the Crimson Hunter (and especially the Crimson Hunter Exarch, whose ability Marksman's Eye increases his damage output) is a great tank hunter. A Hemlock Wraithfighter costs about 30% more than a CH, but assuming you can position it so its Smite hits the enemy, it does about 30% more damage. But a CH Exarch is more cost-efficient than a CH, and slightly outperforms the Hemlock, point over point, when fighting tanks.
Factors in the Hemlock's favor:
1. It's more flexible. The D-scythes are a more versatile weapon, it has the mindshock pod, and it can cast a spell instead of smiting, meaning it can do a broader variety of things.
2. It's better against T8 and T6. The high Str of the Hemlock's weapons mean it outperforms CHs for the cost against T8 and T6 since it wounds those enemies more easily than the CH's weapons.
3. Enemies being hard to hit doesn't matter. Hemlock auto-hits.
4. Mobility. Hemlock can (and probably should) Advance with no penalty every turn.
5. Reliabilty. Hemlock performance does not degrade as it gets wounded.
Areas in which the CH is superior:
1. Fighting T7 tanks. CH Exarch outperforms Hemlock for the price against T7, normal CH matches it.
2. No reliance on spellcasting. Under the beta rules, multiple smites are unreliable. Also, you can Deny The Witch a Smite, but you can't Deny The Witch a Pulse Laser shot.
3. Positional concerns. CHs can hang back, they don't need to come within melta range to shoot normally. Also, a particularly canny player might wall out your Hemlocks with really good screening (though this would be hard).
4. Durability. CHs give you more wounds per point. The Spirit Stones on the Hemlock partially make up for this.
5. Command Point Expenditure. In a crucial moment, you can re-roll a damage roll of 1 for a Bright Lance on a CH. An amazing choice! But on a Hemlock re-rolling a crucial to-wound roll isn't as big of a swing.
6. Working with Asurmen. CHs are Aspect Warriors and benefit from Asurmen's invulnerable save, hilariously enough.