As Jana pointed out, Summoner does have some abilities Arcanist doesn't, like Fester and Painflare. This is sort of the direction Shoshopu has gone with offensive spells from herself, as she can Fester her "contagion magicks" and cast Ruin III, but the primary direction she has gone in is one of investing a lot into her familiars. Most of her geometries focus around building more robust or specialized versions of carbuncles and -egis, and temporarily empowering them with extra doses of aether. They can perform a number of abilities outside of the..... four or so they have in the ingame mechanics. She's also been experimenting with augmenting them, upon their initial summoning, with additional sources of aether outside of her own (i.e. crystals). If I could get her into IC conjury somehow I think things could get pretty interesting with this route.
But in short, her special flavor of summoning was derived from her own interests and improvisations from lack of formal teaching or text, that invests a lot of aether into her familiars to make them more effective on their own while she controls them remotely from a safe distance.
Essentially she's more akin a FFXI summoner, in that her pets do most of the work, while the spells she casts on her own are secondary.
But in short, her special flavor of summoning was derived from her own interests and improvisations from lack of formal teaching or text, that invests a lot of aether into her familiars to make them more effective on their own while she controls them remotely from a safe distance.
Essentially she's more akin a FFXI summoner, in that her pets do most of the work, while the spells she casts on her own are secondary.