In all honesty, a great shot requires a delicate balance of both speed and glide. For instance, if I was trying to anhyzer a drive all the way down the fairway on hole 5 at Morley, I would want the disc to travel as fast as humanly possible, but at the same time it would need a decent amount of glide in order to achieve the longest distance possible.
An easier way to describe it would be to throw a teebird on hole 5, and then right afterwards throw a wraith. Both discs have phenomenal glide to them, however the wraith is faster by 3 or 4 points. If you watch your throws, the Wraith should be clearing the pyramid much sooner than the teebird does. Or in my case (I'm a cheatin' lefty) the Wraith always clears the bushes on the lefty route much faster than the teebird does.
One last example would be a Wraith vs. a Banschee. The Banschee screams like a bat out of hell, however, without any glide it hyzers out and fades much faster than the Wraith, and more importantly, much shorter.
BTW I'm not trying to build up the Wraith as the best distance driver ever, it's not, it just has a great balance between speed and glide that makes it good to compare with other discs. So, to answer the question of which is more important, speed or glide, I'm going to have to say both. Because each hole in disc golf requires a fusion of the two in different amounts for each hole.
For me, the faster a disc gets to where it's going the less chance you have to worry about what the wind is going to do to it. Though for midrange or control shots I'm with CBass on this one, speed kills when you're trying to drop it close to the basket (if your disc goes too fast, it'll just skip right on by when it lands.)
What you're looking for is a disc with the most glide you can get at the fastest speed you can comfortably throw. Throwing discs that are faster than you're capable of doesn't help you any. They'll just fade out early and you lose distance. I've found for me that Speed 10 discs do the trick nicely, which is why Star SLs are my main driver.