Dialga vs. Palkia?

I would say that Dialga is more worth powering up. It offers something no other dragon does in that it takes neutral damage from other dragon attacks. These days dragons are only particularly useful against other dragons and as such Dialga has a niche the others dont. The fact that it uses dragon breath does slightly hurt its DPS but its pretty marginal in most cases. Dragon Tail gets more breakpoints so when it is able to squeak out a few extra breakpoints that dragon breath cannot is when it is hurt the most. But dragon breath is much much easier to dodge with. Thus Dialga offers you a tank for these raids that can not only take more hits but can also dodge much easier. As far as steel type attackers goes, Dialga is far outclassed by Metagross. There is basically no situation in which it would make sense to use a Dialga instead of a Metagross. Metagross has superior DPS and TDO. Dialga has a higher TDO than the other dragons when facing a dragon with dragon attacks. Palkia does better against fire/water moves which would typically be found against itself. Ice is superior for Garchomp, Dragonite, Flygon, Rayquaza, Salamence, Alolan Exeggutor so Dragons are already second class citizens. Giratina does not hit particularly hard so TDO may not be a real issue there but against its dragons move, the Latis, and Palkia with Dragon moves, Dialga is a TDO workhorse that is easy to dodge with and doesn't exactly skimp in the DPS department like more defensive stated Dragons such as Latias or Giratina. I think Dialga is quite nice to have around as an anchor for the team whereas Palkia is generally outclassed except in the case of itself with Fire/Water moves. The thing is that Ray, Salamence (if it gets Outrage), and Dragonite all resist those moves already so their superior moveset probably outweighs the double resistence Palkia gives anyway. Its really the double dragon movesets that cause the issues in the first place since its a 1 hit KO where dodging won't work to begin with. Dialga can probably avoid the OHKO and is easier to dodge with in the first place so it provides extra value. Against Latios its probably not necessary to bring a Dialga because Dragon Claw won't be a OHKO but even then it serves as a great anchor due to its resistance to Psychic attacks as well. Against say Kyurem. Metagross is already resistant to both Ice and Dragon and has higher DPS than Dialga so it is the ideal option even with Rampardos and Rayquaza having slightly better DPS. Rampardos is a better choice than Ray in that situation but Metagross loses less than 1 DPS to rampardos for twice the TDO. Dialga gives neither superior DPS nor TDO compared to Metagross so its a non factor. The fact that you only have 1 Metagross might be a problem against Kyurem. Its also the foremost attacker into Regirock. Regice is another place where Metagross shines but Rampardos is superior DPS wise. Metagross might actually be better overall against Regice as it is not weak to fighting and resists Ice. So that may ultimately outweigh its slight DPS advantage though Regice does not hit particularly hard. The DPS advantage is relatively slight and even in the worst case for Metagross, its TDO is basically double Rampardos. Moltres probably gets the nod in those situations (Rock Smash/Earthquake). Dialga has significatly lower DPS compared to metagross (2 dps less) and will always have a worse TDO due to its weakness to fighting and neutrality to Ice. Against Regirock Dialga is essentially on the same level as Machamp except against Zap Cannon where it manages to slightly edge Metagross in TDO at the costs of a heap of DPS.

Didn't read the above response, but if not answered already:

Dialga is a terrible steel attacker, and by TDO against a fairy/ice, even worse than aggron. This stems from its dragon typing, being weak to fairy and ice (things you will use steel against) causing it to lose out on 40% TDO! It's a much better choice to specialise in dragon.

I want a good one to max out for PvP. Having that short-charge time for the w-bar steel move really helps with the strategy, something that Palkia doesn't have.

I barely care about having the "six of the pentultimate pokemon for raiding X-Type of pokemon on the off chance you are missing a few people."
To me, this is the biggest waste of resources.

I have a few pokemon from each type, and combined they do pretty well. PvP is only going to get better in the future, so I'm pouring my resources into that, that's where the real strategy and skills play in.

In a neutral matchup, DPS-wise, DT Palkia is best, then DB Palkia, then DB Dialga. TDO-wise, DT Palkia is still best, then DB Dialga, then DB Palkia. Use them as dragons or not at all.