So, let's answer a question: is the helmet with bonus to experience + crystal of erebus 8% bonus experience a good choice? You are sacrificing some stats, % damage, health, skillpoints etc. But, aren't levels powerful than stats?
Answer: No, the difference is insignificant. (short answer)
The characters used to do the comparison are both hardcore, xmax, no potions characters. They explored every dark area of the map, completed all side quests, killed the same approximate of monsters (this one is hard to prove as the kills of pets will not count as your kills)First character: Ranger at the end of the hades (epic). He used +12% experience since the days of sparta (level 8 or so) and added a crystal on the helmet with bonus experience at the end of hades normal.
Second character: Summoner, never used a bonus experience item. He only used +% stats helmets and hollowed helmets.
Ranger:
Total experience: 108530613, Level 72 (almost 73 with a bar missing).
https://imgur.com/GtTQu1n - hades kill character screenshot - epic, bonus exp 20%
https://imgur.com/TSuae5w - total experience - epic, bonus exp 20%
Summoner
99382116 experience, level 71 (and a bar)
https://imgur.com/2i5xXeZ - hades kill character screenshot - no bonus exp
https://imgur.com/X4cYAPh - total experience - no bonus exp
Between the characters, the difference is of 9.000.000 experience or, at the rate i got experience on this character, about half of immortal throne act on epic. As for time played, the difference between the 2 characters is of 7.190 seconds (about 2 hours), the no bonus exp character having more time played. The time difference is irrelevant because every time i am taking dinner or go out i let my characters logged in so who knows how much of the time is actual time played. Also, the summoner spent more time on farming vendors for his items while the other got some gear by monster farming.
Let's take a look at another point in time, a point where my character used only the 12% helmet exp
https://imgur.com/TuA4hlx - character screenshot hades kill, no bonus exp -normal (Summoner)
https://imgur.com/7tHehJ2 - character screenshot hades kill, bonus exp 12% - normal (Ranger)
At low levels, the luck our character has with hero monsters spawning weights in more than the bonus experience we use on the items. We can see that best on my 2 characters, compared side by side, where the character with bonus exp has lower level than the character with no bonus
So, for my character, 20% bonus experience accounts for about 2 - 4 hours of play
Conclusion: using only the helmet for bonus experience could impact your leveling in a negative way. By choosing such a helmet (my most lucky find was 13% bonus to exp +8%crystal of erebus) you gain a maximum of 21% experience bonus but you miss on important stats and resistances. A character with a proper helmet would get the same experience or even better by increasing the killing speed and survivability which in the long run will affect the player's enjoyment of the game.
One more argument against using only the helmet for leveling is the chance to get one. I usually got those helmets from vendors, the moments between one helmet and the next one being far. The prime example for this is my ranger which used the gorgon helmet since act1 normal because it was the only helmet with a decent bonus to experience i found.
