Google tells me that Saigon Cinnamon = Vietnamese Cinnamon, which I use for cooking. IMO, good spices are wasted in soap - I buy from Dollar Tree. I really think the subtle differences that make this better than other cinnamons would be lost in soap.
As far as ground cinnamon in general in soap, I like it. It can be an irritant, but so can lots of things. I like the look of it, and it lends a noticeable aroma to the soap, though not nearly as strong as an FO or EO. If I decide to rebatch a bunch of different-colored scraps, I add ground cinnamon and clove because it makes the brown soap look better/more interesting.
My son thinks cinnamon is scratchy even as a thin mica line!
According to a quick wiki search Saigon cinnamon can have more than ten times the level of the toxic coumarin than normal cinnamon so you would have to be careful of your amount used.
Since you aren’t going to get the scent in soap (Dixie says she can detect it but I can’t in my soap) but will get the colour and the look I’d personally stick with the less toxic and cheaper normal cinnamon:
I do notice the texture in it, but I don't find it too sharp or too scratchy. I will say I notice the scent with my nose touching the bar. If you hot process - put the cinnamon in before you cook - then the smell is stronger.