I have the hardest time hitting the glute... can only use one hand and can't steady it with the other because my lats get in the way, or if i reach around the front my chesticles are in the way. BUT when you get it right, you have far less PIP in the glute. What I would do to practice for glute shots is take an empty syringe and pull the amount you would normally in air, and keep the cap on. (WITH THE CAP ON!) Practice the poke, aspiration, and injection slowly so you get the feel of doing it with one hand.
With the quad shot you can zoom in nice and close so you can see what the hell you're doing and hold it steady with the other hand.
I would say at least try to hit the glute. But quads are for sure easier.