If you were to run a Gothorita, chances are it would hold an Eviolite, this means you loose a held item like leftovers. Eviolite can also be knocked off, crippling Gothorita. However, thanks to Eviolite Gothorita has higher defense and sp. defense than Gothitelle.
With max evs and ivs at level 100 with a calm nature their defensive stats would look like:
Gothorita (with eviolite) Defense: 358 / Sp. Defense 422
Gothitelle Defense: 289 / Sp. Defense: 350
Gothitelle also has higher speed and HP by 10 points and higher sp. attack by 20 points on its base stats. Gothitelle can also hold an item like Leftovers or Light Clay. Given their hidden ability is Shadow Tag, you might want to economize on that with Toxic. Gothitelle is better suited to do damage with its 95 base sp. attack to speed up Toxic's toll. These Pokemon only have one self healing move; rest, that Gothorita can use this to stall out the opponent, but so can Gothitelle, if less effectively.
Given this analysis I believe that Gothitelle would be the better choice, because it can hold an item and is more versatile. However, if your looking for strait up bulk, go with Gothorita, just watch out for Knock Off, Switcheroo, and Embargo.
Thanks to sumwun for helping me give a more accurate answer!