Hi,
It also depends on gears u have no on that summoner.
Invokers are still healers, most of their buffs are quite insignificant. Better the staff u have, meaning higher your dmg is faster u will kill.
Next thing is attack speed, this is also the most important part of both summoner and invoker. I don't know about summoner, but invoker has a speed cap around 258% on stable 60fps.
3rd thing is actually dmg diffenrence, summoner is alot stronger at dpsing. On the other hand u need stuff like HP/mana pots to keep yourself healthy.
As for invoker, their dmg isn't very high it's still more than enough for farming, u can clear solo any content. HP pots aren't needed since healing skills which are based on your max HP, not magic damage, are strong and fast. As for mana, most of the attacking will be spaming x, u can have some mana pots but in my experience u don't need any.
So in short it depends on you, both summoner and invoker can do the same thing, the difference is playstyle and speed in killing mobs, with nicely geared summoner u will clear anything, with nicely geared invoker u will clear anything. So in total, choose whatever u like, both classes are OP.
I like invoker since I have one, so this post might feel one sided
