There isn't really a difference. I was speaking to a man today who makes custom bow ties for a shop in Prince's Arcade in London. He said that Faille is a textured silk, generally ribbed but occasionally just with a texture. Grosgrain is essentially the same but is generally not used for bow ties as it isn't as sturdy as Faille and most of those marketed as Grosgrain are in fact Faille.
My (slightly impractical) suggestion would be to go in with your dinner jacket and match it as closely as possible as that's what matters not the material. Alternatively if you had the dinner jacket made for you (I'm guessing you did if you're interested in a custom bow) ask them if you could buy the same silk they used as that would produce the best result!
One other tip consider a pointed end tie if you have a peaked lapel jacket, I've found the points harmonise well with one another even if it's even more fiddly.
This may be a bit late but good luck!