-Immune to Terror
-Provides extra resilience
-Can trigger No Mercy
-Slightly underwhelming stats.
-Very threatening on its own.
-Discountable thanks to Enclave and Bonesingers.
-Great synergy with attachments and other cards (thanks to Drone trait).
I would pick the Wraithblade in most cases. Shadowsun wants to commit where you already have units, so the only weaknesses it has is compensated since the warlord should win the command struggle there.
Unless you're playing in a meta completely dominated by DE, I wouldn't pick the Fireblade.
Or you can simply go one of each and avoid thinking
Concur with this post, especially the bolded part.
For a while I felt like the lone voice in the wilderness, pointing out that Black Guardians was actually really good for Shadowsun, because of its high HP base giving somewhere to safely put attack enhancements, and the nature of Shadowsun's game mitigating its command weakness. I don't use Black Guardians any more, because Wraithblade gives so much more bang for its buck.
The only caveat here, and worth adding to the CONS list above, is that Wraithblade strongly telegraphs your commitment to that planet. You can play clever feinting games by putting it to planet 1 and then sending Shadowsun elsewhere, but doing so "reactivates" its command weakness and is a high risk game.
Also, from a strategic point of view, I'd say Wraithblade and For The Tau'va really are best buddies.
You'll often find that you get a planet given to you by a Wraithblade to planet 1, and then the opponent feels a bit safer because it is coming in exhausted. Messing up that calculation is very, very powerful.
Incidentally, I find it very odd to be comparing Fireblade to Wraithblade. A much more sensible question is Shi Or'es vs Wraithblade, as they are far more comparable in function.