Lol, I'm not really too sure about this myself!
I REALLY wish that I did have access to my Wii.
Yeah, apparently MK's ^B doesn't go at a fixed height. Think of it like Pit's and Snake's ^B. Again, I've got another thread to show you on this:
http://www.smashboards.com/showthread.php?t=217069
I haven't really read through all of it, but I'm pretty sure that the outcome was that if you jump before using ^B, he'll jump at maximum height. Again, I can't try this for myself which sucks ***.
From what I remember, the glide does cancel the vertical momentum, but only a small amount horizontally because he is still moving back slightly as he glides. Although, the difference between the 2 options isn't that great as I stated earlier that it was only a 5% difference. However IMO, the ^B option is the easiest one to perform reaction wise compared to the other option. So I think people would be more obliged to use that. Although, the jump into a glide has it's uses too, especially if you aren't at a disastrously high % where you won't gain a lot of glide height to return on stage which will make you easy to intercept plus you have the helpless state to deal with when you cancel the glide once you are on/above the stage.
Yeah, it's evident in that video that the ^B is better than the jump into glide. The ^B prevents MK from reaching the highest platform where as the other option doesn't. Although, the difference isn't that great as you can clearly see, it will only increase your survival by about 5%. So both options are viable, however as I stated earlier, the ^B one will probably be the easier one to perform.