Green Velvet is the hardier of the two, surviving at or nearly at -30 here in Iowa. Green Mountain would be better if you were looking for a hedge that can be pruned into an upright or conical fashion as it is more pyramidal compared to the rounder Green Velvet. Green Mountain may work for you in 5b, but where we are here (a solid 5a with sometime 4b temps or colder), Green Mountain will die back to the snow line once it reaches below -20 to -23. If there is a chance that you will reach these temps consider something else. Also, be aware that there is a boxwood blight circulating the Eastern U.S. and becoming more prevalent in the Midwest. Your plants will need to be certified blight free. Most production nurseries have started doing this but check just to be sure.

I cannot assist you but did you try Rice Road Greenhouses near Fonthill/ Niagara Falls ? http://www.millionplants.com/products/hardy-tropicals/ I personally saw that Jim Lounsberry of Vineland Nurseries ( just outside of Vineland ) had : Fargesia denutata and Faragesia rufa cv.

I think many conifers grow very slowly. Pine trees are the exception. They grow very fast. You may consider those, but they are wayyyyyy more than 20ft. Another fast growing conifer is the Witchita Blue Juniper. It will get 4-8 ft wide and 15-20 ft tall. If you plant them, make sure to plant 4 feet next to each other. It can be used as a screen or a windbreak and will need no pruning. It also has appealing color. Goodluck with what ever you choose.

Slower growth than I expected. Pretty healthy; not sparse or spindly, good green color. Growing at the top a slope (drains fast) and gets very little summer watering. At the rate it's growing, I'll be long gone from this world before it makes a privacy hedge, though!