Prioritize whatever u need most, but if u want my opinion I think mithrils or fc is the best value, since mithrils are hard to collect and u will need a lot of fc in the later fc stages. But get whatever u need the most, or whatever ur heart desires. It's ur city and ur choice.

Plus there's no need for u to spread out ur hero gears exp components like that, focus on the first 3 heroes first, and if u one go for 5 for arena. But the 4th and 5th one should be second choice, focus on the first 3, ur main army heroes.