Heres a couple of things to consider;
Both of these plants equally like a high light tank and a rich substrate.
Hairgrass spreads much slower than glosso, so if you want this stuff to spread and cover a large area be prepared to pay a lot of money for a lot of hairgrass. Glosso spreads fairly quickly, a little goes a long way. Hairgrass can take 2 months before you see any visible spread. Glosso will begin to send runners within 2 weeks and within 2 months ,the planted area will be completely covered in glosso.
These have been my experience with these 2 plants. That being said, I love both of these plants. Why not get some of each.
FILSTAR Pimp #2
75 gal heavily planted,50/50 Black beauty,Eco-complete substrate, Pressurised CO2 with solenoid, ph controller, AB Reactor 1000, 330 watts 9325K GE PC lighting,Ehiem Liquidoser, 2-Filstar xp3 canisters.