I'd go with harmony if you can get it in a matri bow.
On a lower budget the lightning skills will be a better killer then your bow, and harmony's aura is useful even while you're not encountering any LI's. WWS on the other hand only take care of the immunes (be it gallantly, but still ...).
Harmony should therefore prove more overall useful, unless you're running areas with lots of LI's.
But really ... if just personal preference, both work fine.
If you are on a budget, it seems like a good idea to go for the high 'customibility' of the wws. Bow's need ias, knockback, leech... You can fill in the sockets on the wws with what you are missing, with harmony you are stuck in what it offers.
That being said, if you can get the needed gear together - ias, knockback, leech... just make sure you are happy with your bow side - , harmony would be my choice.
Well Kijya, I'm sorta using a bit of your gunslinger guide. Just gonna be hard to get that Mat bow + 3 skills + 4 sockets and even then, I might as well use it as a Faith.
NASE - I'll be using Witchy until I can get harmony. :-)