I mean, Haste is also concentration, but I guess it has defensive benefits that will help keep it up.
The main difference between the two I'd think is that Haste does not need to be upcast, whereas Shadow Blade does need to upcast to scale. However Shadow Blade probably scales better than the...
Seeing as I didn't see it discussed in the guide and searching the thread turned up no results, have you done the math on 1handed builds using the Shadow Blade spell? Especially in comparison to the Haste TWF build.