The distance Diddy Kong travels during a glide-toss is not dependent on the direction of the throw but rather the frame where the roll transitions (interrupts) into the item throw animation.
[collapse=Throw Direction Comparison]
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The above throws are done at the quickest possible timing (1 frame into the roll). Notice that Diddy throw animation ends with him just barely beyond the MasterBall outer center circle.
[collapse=Toss Timing Comparison]
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You have a 4-frame window at the beginning of the roll to input the item throw to achieve a glide toss. The first gif shows the distance achieved at the earliest possible timing; the second one shows the distance at the latest possible timing (4th frame). Essentially, the later the interrupt the longer the travel distance.
As far as its utility vs throw out of a wavedash it should go without saying that you'd be sacrificing a much quicker throw for moderately greater distance.
[collapse=Wavedash Item Throw]
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This would be the max distance considering ideal control stick direction.
Some data for the earliest possible time the banana goes airborne and is capable of hitting the opponent from both throw variations, directly from either standing or Out-of-Shield:
Forward/Upward Item Throw:
- Glide Toss: 8
- Wavedashed Toss: 22
Backward Item Throw:
- Glide Toss: 9
-Wavedashed Toss: 23
Downward Item Throw:
- Glide Toss: 7
-Wavedashed Toss: 21
EDIT: Here's a clearer portrayal of UpB's hitboxes and intangibility. Keep in mind that UpB has 4 separate subactions or parts to it; here they are in order:
-Startup
-Charge (holding B to gain height)
-Active Frames (starting when the hitboxes emerge through the apex of the boost)
-SpecialFall (when Diddy falls with his hands high in the air, unable to act until he lands)
Note that you can bypass the charge portion completely by simply not holding B by the end of the Startup.
[collapse=Startup + Active Frames (No Charge!)]
Up-B
Grounded
Intangibility: 1-4
Hits: 5-7, 8-16, 17-39
Aerial
Hits: 8-10, 11-19, 20-42
Landing Lag from SpecialFall: 8
Landing Lag from Active Animation (while jetpack is still out): 16
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Note that the hit timings above are without any charge, so hits will of course come out later depending on how long you charge the attack.
[collapse=More In-depth Look]
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The hitbox sizes are a bit misleading when viewed only from the front. As you can see, the hitboxes attach to the jetback barrels now and thus emerge slightly into the Z-axis (behind Diddy Kong). Even though Smash is played on a 2D plane all hitboxes and hurtboxes are 3D, so for example during the later hits an opponent's thinner hurtbox (an arm or leg) can overlap with Diddy's arm and still not take a hit if the arm or leg doesn't veer a bit into the Z-axis as well to overlap with the barrel hitboxes.