Ultima vs Decisive Pumpkin

Introduction

The main difference between the 2 Keyblades are their abilities:
Decisive Pumpkin has Combo Boost, which makes ground finishers stronger the more hits you landed during a combo.
Ultima has MP Hastega, which speeds up your MP regeneration by 75% as soon as you used it all up.

A picture showing the stats of both keyblades

Difference:

1. Damage

The 2 Keyblades have been compared with a normal ground combo against Data Roxas, who has 3168 HP.
This was tested with a Lvl.99 Sora who has 80 Strength and 64/67 Magic. (The magic varies because Ultima has 3 more magic stat than DP)
One Combo Boost equipped, no Combo / Air Combo Plus.
Tested was a whole combo attack, with 2 Default Hits and 2 Finishers.
First Roxas was blocked, then the following attacks commenced:
Counterguard -> Flash Step -> Guard Break -> Explosion

Roxas' HP afterwards:
Decisive Pumpkin: 2925 HP (243 Damage)
Ultima: 2957 HP (211 Damage)

A Picture for visual comparison:

A picture showing that Decisive Pumpkin does more damage in one combo to roxas than ultima does

Decisive Pumpkin did 32 more damage than Ultima. This doesn't sound like much, but it makes the fight much quicker. Air Attacks will still do the same damage. The 3 extra magic that Ultima has barely makes a difference during a ground combo, because the only attack that does magic damage is "Explosion" (The balls that circle around you).
Another comparison was done with the Lingering Will boss, with 2 Combo Plus and 2 Finishing Plus abilities equipped.
A full 4 hit ground combo with 3 finishers, which consist of Guard Break -> Explosion -> Guard Break . The following picture shows his HP Bar after a full combo with both Keyblades:

A picture showing that decisive pumpkin does more damage to terra in one cycle than ultima does

Decisive Pumpkin did a lot more damage than Ultima, and that with the same attacks and finishers.


2. MP Regeneration

The only thing Ultima does better is MP Regeneration: With Decisive Pumpkin, your MP bar needs around 22.5 seconds to restore. Ultima needs only ~17 seconds to fully recover your MP, which is way faster than any other Keyblade. These times where measured with 2 Full Bloom+, MP Haste and MP Hastera equipped, so your times may vary.
A video comparison between DP and Ultima (reload page to synchronize):


3. Keyblade length

Ultima is slightly longer than Decisive Pumpkin. It's barely visible, but it's definitely longer. This doesn't make much of a difference, but it should make connecting attacks easier when dashing in with 'Slide Dash'. It also makes Dodge Slash a little bit more viable. A .gif showing the difference:

Conclusion

Overall, Decisive Pumpkin is a better Keyblade for doing a lot of damage with physical ground combos, but Ultima is better if you prefer to take it easy and heal a lot.