The next two units are nominally Elites, but can be taken as Troops if you take a Spiritseer in the army. Since the Spiritseer is a good unit and T6 scoring units are awesome, I expect you will see them as Troops more often than not.
Wraithguard are tough as nails. With T6 and a 3+ save, they’re as hard to wound as most monstrous creatures. Their downside is that they lack Battle Focus and Fleet, making them quite slow for Eldar. This can cause problems when paired with their short ranged weapons. Their basic gun is the wraithcannon, which is S10, AP1 and the Distort rule gives them a chance to inflict Instant Death or auto-penetrate a vehicle. That’s really terrifying, but it only has a 12” range so they’ll have to get quite close to the enemy.
Alternatively, you can pay to upgrade their wraithcannons to D-scythes, which are AP2 flamers that have Distort, same as the wraithcannon. It’s a bit shorter range than the wraithcannon (but only by 4”) but it will tear through any units in range. Even monstrous creatures and heavy vehicles need to fear it because it can automatically kill them. To top it off, it’s hard to imagine any unit assault a unit with D-scythes and survive their overwatch.
Wraithguard can engage just about any targets. Those armed with wraithcannons being better for taking out vehicles and monstrous creatures, where their S10 can do the damage without relying on their Distort rule. D-scythes should concentrate on murdering infantry, as well as holding objectives. I think either type of squad benefits from a Wave Serpent to get them into range of the enemy, but larger squads are durable enough to advance on foot.
Wraithblades are one of the new Eldar units, and are essentially Wraithguard armed for close combat. They possess the same durability as Wraithguard but sacrifice their terrifying guns for power weapons. They can either take a pair of power swords or a power axe and force shield (granting a 4+ invulnerable save).
Frankly, Wraithblades aren’t a very useful unit. They’re an approximation of Assault Terminators, but paired power swords and the power axe/forceshield combo are inferior to paired lightning claws and thunder hammer/storm shield, respectively. Beyond that, Wraithblades only have 1 attack base, so they aren’t throwing the number of attacks necessary to be a real assault unit.
These deficiencies could be forgiven because Wraithblades are quite a few points cheaper than Assault Terminators. However, Assault Terminators have access to Land Raiders and Stormravens, both of which can deliver them directly into assault. Eldar don’t have any assault vehicles, which really robs the Wraithblades of the mobility they need to be a viable assault unit.
I hate to say it because I love the models, but if you want to use a Wraith unit to tear apart enemy units and take objectives, take Wraithguard with D-scythes. They’ll be far easier to use effectively.