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I think the decision varies from person to person. If it fits your character's personality and build, or your mood and curiosity, I say go for it; sounds great.
For my character, the narrative supports her either curing lycanthropy and being human again, or staying a werewolf basically forever. Vampirism really isn't in her cards. Neither is assassination. So, for me it would be a bad plan, but only you can really determine if it works for your character and play style.
Werewolves make for better assassins, unless you can become a Vampire Lord (Dawnguard DLC).
As a werewolf, you can shift in a private area, run at the target, kill them, and then run. So long as o one spots you shifting, you're fine.
As a base vampire, you'd have to rely entirely on stealth to get the assassination done. However, you could pull the same thing as a VL as I said you could with a werewolf, and be able to shift as much as you please.
VL requires the Dawnguard DLC though, so if you didn't get hte LEgendary double-disc edition you're paying for the VL while the Werewolf is free. I'd say stick with werewolf.
You can be switched back by the companions, but I kind of look at this way: when you go vamp your undead and so there's no going back.
The way I'd play it is complete the companions quest line (stay a ww) and then abandon them. Follow the dawngaurd quest line and let Harkon change you and start a new 'life'.