is 20-25 active skill increase a noticable change or a rings trap
In my humble opinion, ut depends on what level u r at. It is not much of a ring trap if u r high level already. Yes, the number of rings to get to level 25 seems daunting but rings r easier to get now... if u r like me, still trying to get ur first evo, it is not necessary. All my active has their active skill to the lowest cost. Hope this helps.
It will really depend on the skill.
Some skills get to their lowest rage cost past lvl 20.
Some get a bonus to the skill after lvl 20, in either damage or effect or count, etc.
Some aren't worth the time at all.
Wolvenfiend will pretty much become god like (if it weren't for the nerfs) in both the survival as well as damaging aspect. Skill 21 is the bare minimum for a semi good tank and damager. From that point on, it goes exponentially. He does much more dmg on skill 25 than he does on skill 21 when in Wolf form, and he will soak up a lot more dmg when he's skill 25 in human form than previously
Others are simply not worth it as mentioned by others. Abyss Demon on skill 20 is what 1 should aim for, as all of the next levels in his skill will only result in a 2% self healing increase per level as opposed to the 4% self healing per level going from skill level 1 through 20.
Seek out the exponential effects and you'll be good
Wolf is the only hero that I have skill 25. It makes a big different since his skill is more important than his divine. However, it costs alot of rings. From lvl 1 to lvl 20 costs 41,350 rings. From lvl 21 to 25 cost 40,000 rings.
It is easier to get rings but it costs alot of rings for upgrade 5 heroes, plus 15 aids! For my opinion, hero aids are as important as main heroes. I invested my 5 berserker aids to lvl 20, and it is worth for the costs.