I use both. QL, for example, is dead-nuts on with my .223 loads while GRT is off. GRT has been much better with 6mm GT. This is feeding both with accurate case volume information.
GRT is generally easier to use, but does not have integrated exterior ballistics. QL is under active development.
So plusses and minuses to each. But each of them are useful for choosing bullets and powders, which is my main purpose. Actual velocity I'll measure with a Labradar LX anyway.