They're pretty close.
Dame's a better outside shooter. He's a slightly better free throw shooter. He's a better rebounder.
Isaiah is a better finisher near the basket. He's a slightly better defender, which means he's a much more focused / high effort defender given the size difference.
Also, they both have looked pretty overmatched trying to carry a team without a lot of help in the playoffs. Isaiah especially so (through 6 playoff games he's 33% from the field).
Overall, if you were choosing one or the other to build around, you'd have to choose Dame because he's younger, slightly better overall, and likely to play at a high level for longer.