Special rend: The high critical hit ratio makes the 90 s. atk into 180 s.atk, which occurs 12.5% of the time. so ((90*87.5%)+(180*12.5%))/100% = 101.5 dmg. It has an accuracy of 95%, so 101.5*.95= 96.18 per turn.
Roar of time: The critical hit ratio makes the 150 s. atk into 225 s. atk, which occurs 6.25% of the time. so ((150*92.75%)+(225*6.25%))/100% = 153.18 damage. 153.18/2 for the "recharge" turn is 76.59 per turn. It has an accuracy of 90%, so 76.59*.90=68.93 per turn.
becuase both Pokemon have equal base sp.atk, and both receive a STAB benefit from it, the comparison is accurate.
An unbiased answer, as is the following info.
Remember that dialga has resistance to 11 types, and is unaffected by one. He is weak to two types, which are fairly common (ground and fighting).
Palkia is resistant to only 3 types, and is weak to one uncommon type (dragon).