As someone that plays both games, here's my personal input:

King's Raid over E7 ~Auto Friendly Grinding, less babysitting the phone ~No Hero Gacha ~Almost all the heroes have uses so you can build many and not feel like you're wasting resources ~Multiplayer content (I play with 5 friends, it's pretty cool to run dungeons and challenges together, plus nice guild content) ~Costumes (Ok, I'm a girl. I admit it) ~Relatively F2P friendly. 2 of my 5 friends don't spend at all. They still compete but you have to be more selective on heroes you build and gear ~Ways to guarantee you get the gacha items you want (unique weapons and treasures can be gained from in game content, albeit slowly.) ~No stupid charge to remove your gear and put it on another character ~Very little power creep, it exists... but old characters aren't useless. New characters aren't always better than old either, just different. ~No Element based combat system. Just Magic and Physical (I find Elements restrictive, especially for games that have hero gacha like E7)

E7 over King's Raid ~Main story is better written ~Animations are Flashier ~More events/More generous events (new game, KR was more generous previously but they aren't bad now either) ~PvP isn't necessary (You'll slow your progression for KR if you don't pvp at all, but I guess that's also sort of true with E7 because you'd miss out on skystone?) ~Summon rate is a bit higher, but this is likely because you're pulling for heroes and artifacts. The impact from summoning a shiny thing in KR is much higher though. ~Better optimization on older phones. My S7 does not like playing KR when there are lots of mobs on screen. ~Hero Recall system when they nerf heroes is really nice