Both. I used the rank up gem to take Kitty to r5 and have Bishop at r2. Bishop gets slept on as an attacker (outside of those pesky skill champs that shrug debuffs). Both are good defenders as well. There's always room in your roster for an r5 Kitty even with the shift to 7 stars.
If you don't use Bishop, then rank your Kitty. She'll clear more content and I'd imagine she's already awakened and at a high sig, and won't be released as a 7 star anytime soon. I did the opposite, but I also don't use mutants as much as other classes, so I went the 7star route first. Keep in mind that rank up gems are becoming more common, so you can use your rank up mats on higher rarities.
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If you don't use Bishop, then rank your Kitty. She'll clear more content and I'd imagine she's already awakened and at a high sig, and won't be released as a 7 star anytime soon. I did the opposite, but I also don't use mutants as much as other classes, so I went the 7star route first. Keep in mind that rank up gems are becoming more common, so you can use your rank up mats on higher rarities.