Level up Sunsinger first, because self-res is gloriously useful in activities like Nightfalls, Hard raids, and doing things solo on higher levels. Then switch to Voidwalker, because it's a more fun class to play in my opinion. Also, it takes time to get used to using Glide. Its easier coming from Lift, rather than Double Jump, but it's still a very different jump. You maintain velocity (speed and direction) so to go up with Glide, double tap the jump button to activate Glide right after your first jump, and you'll go up. Depending on how long into your first jump you wait to start your Glide, you'll stay level, or go down. Don't worry about starting the Glide too early in order to go up, it lasts almost twice as long as Lift. And the "initial boost of momentum" perk for it is definitely the best.