Azure, rusty, and other super dragons: do they live up to the hype?

Okay, but let's say there are two Dungeons available. For 15,000, you can get 28 Eyeballs and 16 Medusas, and still have some resources left over. Or, you could get 1 Rusty Friend and also 14 (!) Sulfur (!). Which is better? Maybe the Rusty Friend is indeed a good choice... but... Sulfur isn't infinite either, and you might also encounter a Bog Fiend...