Why not use all 3?
Push the Limit: More MH% and OH% damage at once by x%. This is massive. For example, if you have 750% meteor damage, with 50% push the limit, you can increase this to 1125% meteor damage. With hero point 20 combined, 2250% MH% dmg. This can also apply to cosmic orb and offhands like torrents, etc. Especially procs to. Increase your MH% and OH% but at increased skill cost.
This applies to primary skills and secondary skills of MH% and OH% as well as procs and mythics on gear. It works kind of like how Identity adds 62.5% to skills of different class but 50% to skills of any type.
Glasscannon: more DMG output but less HP. 50% glasscannon means you will deal 50% more of all damage output but your HP will be cut by 50%.
Barbarian; Similar to glasscannon but if you have 50% barbarian, cuts your MP by 50% but you deal 50% more overall damage output.
Barbarian and Glasscannon are like the legend affix pure damage boost before the sets like momentum and demonic for example but work a similar way.
Push the limit works like how hero points increase skills. It's even more significant with Discordance. For my orb build especially like my PvP and PvE version like this one. It's a must have in my opinion unless you can't have it due to MP running out reasons.
This build here uses all 3 tbh.
In PvP, it's a different story as it's dependant on what build you make. Glasscannon even though at 18% is still significant at reducing your HP by 18% but if you don't mind that, 18% more DMG on a slot.
Barbarian you can use in any PvP build but you will halve MP by 18% in PvP. If you can deal with that, 18% more DMG. It's a nice alternative to glasscannon or you can combine the two if you like.
For push the limit in PvP, well that's commonly used but you increase your MH% and OH% of all procs, mythics, Primarys and secondary skills meaning all your damage dealers are increased by 18% and your damage output becomes so much more. Even 18% is enough but 36% meaning 2 PTL affixes is even better.
But if you can use all 3 in your build and still function very well in PvE and PvP or one of the other, then you can go for it.