As someone who has flown both, and instructed in Gliders I would say - one at a time. They are both different forms of flying which require different disciplines. Unless you've spent all your life in an aviation background and have a solid (like, you eat, breath, sleep and work flying) then you're just going to get confused.
Solo in the glider first, consolidate a bit, then do the PPL - what you learn in Gliding will hold you in good stead.