here are the Pros and Cons of each form

Base Form Dialga
Has access to any item since he doesn't use the Adamant Crystal which is often better since raids and normal battles are tougher without items like leftovers or a Shell Bell
pros
-can hold items
-has the same stats as Origin Forme Dialga except Sp def
Cons
-has lower Sp def then Origin Forme Dialga Which may or may not be what you're looking for

Origin Forme Dialga
Origin Forme Dialga has higher Spe Def then normal Dialga but it's only by 20 points which isn't that great and without a held item to help him heal in battles he might faint rather quickly
Pros
-higher Sp Def
-Cooler design (my opinion)
Cons
-has absolutely no access to healing which is a problem
-needs a held item to transform which limits a lot of options
I would personally recommend normal Dialga, as that 20 plus Sp Def in exchange for your held item isn't the greatest trade, and you have no healing in raids and battles except for potions in battles and heal cheers in raids
Source: personal experience with each form