The math I'd out there, or you can calculate it yourself.  😀

With DD being the biggest energy constraint I will be running solo spider20 (also need to refill silver from artak path) as the training is easy for me. 12.3 provides significant more xp and also greenies for summon rush and chase.  So balancing out your silver, xp, dd points against your energy and other training resources is the plan.  If you have tons of expiring energy and ample silver, running some campaign to save brews in training can be a bigger chunk.  If you will be scraping by on energy, go all in on soloing d20s.  I wouldn't consider your d25 option at all.

Don't undervalue the mystery shards if you are tight for summon/chase.  The event overlap currently gives you good options right now.

i run stage 25 solo with orn and 4 chickens, the xp is decent but of course not as good as doing campaign mode, but you get the added benefit of being able to farm actual good items while leveling chickens, so if ur not in a huge rush for the chickens you might as well lap up some potential items and kill 2 birds with 1 stone at the same time.

Campaign 12-6 is 8 energy and gives me 8800 exp so 16 energy for 17600 exp 

Dungeon 20 is 16 energy and gives 5999 exp 

Dragon level 20+ all give the same experience which is 14454 total. The other dungeons are similar, but you'd have to look up the exact XP reward unless an approximation is acceptable. Campaign 12-3 brutal gives 17304 total, and nightmare 38415 total. This is divided over your team, but you'll typically have 1-2 maxed farmer champions that won't benefit from their share. Formula for the effective XP gain per energy for any area would be

R * (T-F) / (T*E)

Where R is the total XP rewarded, T is the size of the team, and F is the number of farmers used.

So for example, 12-3 brutal with 1 farmer would give you 17304 * (4-1) / (4*8) = 1622.25 XP per energy spent, dragon 20 with 1 farmer would give you 14454 * (5-1) / (5*16) = 722.7, and dragon 20 with 2 farmers would give you 14454 * (5-2) / (5*16) =  542. Farming 25 with a duo would be quite inefficient, as it gives not only less dungeon diving points per energy compared to level 20, but also the XP per energy would drop to 433.6.

So obviously a hit, but you get more silver and dungeon diving points instead. I don't remember the exact numbers, but IIRC, dungeon 20 gave about 50-60% more dungeon diving points than 12-3, so whether you should farm campaign or dungeon 20 is more or less determined by whether you are in more dire need of dungeon diving or champion training points and we're not taking gear, silver or mystery shards into account.

Yeah, basically what EGDNIT said. My back-of-the-envelope is about 3x more XP per NRG in campaign.