I'd play Pokemon Platinum. Being able to still hack the events like Darkrai or Shamin (one of my favorite Pokemon events ever) is a huge benefit for the older games.

I'd say get ultra unless you care about deep Pokemon lore. The gameplay in the ultra games is just objectively better. Small QOL stuff like Emerald vs RS, but it's still just simply better in some ways. However ultra changes the stories of the games for the worst, like it just doesn't make sense at all.