Have a look at your stats and see which one needs the boost the most, that is what I will do but troop stats is normally the one to go with as that is the most important. I just upgraded my Buc pistol to level 5 as I needed all the attacked boost the most. Also did a transformation , bought the 30 smelting substance from the event only to find out it only gives part of the boost and need to do much more to achieve the full transformation, wow. and if you buy the smelting substance from packs 99.99 bucks for only 90, more money for GOG and I find in this corona virus time GOG could be more sympathetic to players instead of pushing more spending, give better rewards is the way GOG should go.