you will do better without trans because more Magic Attack.
DK always has a 10% forced damage reduction.
Even stuff that is fixed damage like black dragon's 100 dmg flying fire will do 90 dmg. You can even survive any girgash's meteor if you transform as DK and have full health.
DK is better than paladin if you have clouds. If you're not close to 1500 stats, you can reroll in rabbie arena for 360 of your favorite stat then keep it indefinitely. If you have no clouds, then it's a toss up and who cares since either way it's only gonna last a few minutes.
If you're endgame using /any/ trans can lower your damage, in which case you would only be using it for stat regen or as an "oh shit" button. DK performs better as a stat regen (due to more stats) and as an "oh shit" button (due to better RNG-free tanking).