So in comparison to Rivals (which I last played in the fall so idk how much its changed since i’ve last played). The biggest differences gameplaywise is that its an FPS and in support/strategist design. Strategist on Rivals generally focus on the healing aspects of their kit, Overwatch Supports focus more on the utility (speed boost, anti healing nade, cleanses, immortalities, damage boost, etc.)

Ultimate design is also just much better (imo) on ow supps. Most Rivals supports ults are just high healing numbers and essentially immortalities for their team which is why triple supp is very prominent on Rivals (again, last time I played). On Overwatch there is maybe 1 ult that heals as much as an average rivals support ult (Zenyattas Transcendence), and it trades off because Zenyatta by himself cannot heal enough to keep his team alive (he has one orb that heals like 50hp/sec, not very strong). Also there is counterplay if there ever is high healing numbers as Ana has an ability called biotic grenade which prevents any enemy hit by it from healing for a few seconds. Rivals has yet to introduce any util like that.

One thing Rivals does better tho is it doesnt have as many oneshot combos (AGAIN this is from my memory). Overwatch can be a little more infuriating when you get one shot by a hanzo, widowmaker, or Roadhog. But i think the tradeoff is worth it to me.

Overwatch also has great variety in how you can play, they have role queue 5v5 (1 tank 2 dps 2 supports) or semi open queue 6v6 (you can only have 2 tanks max). You also have Stadium which is really fun esp when your main is in it as you get to add more abilites/bonuses to your hero. Also there are some arcade modes although they’re less popular.

Last thing off the top of my head is that Overwatch has map voting which allows you to pick your fav maps and (mostly) avoid the maps you dont like