The aim of this topic is to try to help you decide whether to get Diamond (for Dialga) or Pearl (Palkia.)
A noticable difference is in the Base Stats. It is a choice between the speed of Palkia and the HP of Dialga. You have to choose whether you want to risk not attacking first or make yourself vulnerable to critical hits. The other base stat difference is that Palkia has better Special Defence, however Dialga has better Defence. Again, choose which you think suits your style best.
To counter Palkia's poor(ish) HP, you could attach a Leftovers by catching a Munchlax or finding it on Victory Road, recovering HP each turn.
Alternatively, you could attach the item that makes you attack 1st to Dialga (I forget the name.)
There are item differences, Adamant Orb only appearing in Diamond and Lustrous Orb in Pearl, as well as some items being more common in Diamond than Pearl and vice versa.
Another difference is that Dialga cannot learn HM 03 Surf while Palkia can due to it being a part water type.
There are also numerous TM differences- visit
www.seribii.net to get the lowdown.
Dialga move list
Basic Dragonbreath Dragon
Basic Scary Face Normal
lvl 10 Metal Claw Metal (Water Pulse Water in Palkia)
lvl 20 AncientPower Rock
lvl 30 Dragon Claw Dragon
lvl 40 Roar of Time Dragon (Spacial Rand Dragon)
lvl 50 Heal Block Psychic
lvl 60 Earth Power Ground
lvl 70 Slash Normal
lvl 80 Flash Cannon Steel (Aqua Tail Water)
lvl 90 Aura Sphere Fight
Ideal Moveset (in my opinion)
Roar of Time/ Dragon Claw
Heal Block
Slash
Aura Sphere
Choose Roar of Time if you want a hard hitting attack with low PP and lower accuracy or Dragon Claw if you want more accuracy and PP but less damage. They are both the same type so there is no point having both incase of Dragon resistance.
For Palkia, where moves differ, check out if the accuracy, damage and PP differ in any way.
Ultimately it is up to you so choose which Pokemon you prefer and get the correct version. I hope this has helped you decide which version to buy.