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I already have 10 Wraithguard (1 Squad of Cannons, the others with Scythes), I have a box of unassembled Wraithguard/blades and I am wondering what option you think I should go for.

If you want to play for fun, go with WraithBlades (I prefer the axe/shield). Your opponents shouldn't cry cheese vs Wraithblades.

If you want to go hard-core win, go with another Squad with D-scythes.

I have a unit of wraithblades with axes and definitely not the optimal build with the new codex. However, if your Guard are walking, they can act as a nice line of cover in front utilizing their 4+ invulns while giving a cover save to those behind. If you take them in a CAD with a warhost detachment the ability to run 6" every time gives them some reasonable mobility.
I currently have 4 units of 5 wraiths...2 wraithcannon, 1 scythe, 1 axe. I would probably take a second cannon in your case. Cheaper than the scythe and more dangerous with the D chart.