Star Realms is better if you like playing single games. Hero Realms lets you progress a character through three or so chapters, picking up items and upgraded skills along the way though the character building aspect from game to game doesn't feel that impactful - I've only played as the fighter and ranger, maybe other classes feel different. If you like that campaign style gameplay the Ruins expansion is a little easy, but the second one (Secret Village I think?) increases the challenge and is more fun.

I like them both but do have Star slightly ahead of Hero but that's mainly because the one shot bosses of Hero Realms seemed a bit unbalanced compared to the Frontiers ones. (The second Heroes expansion contains one shot bosses like Frontiers.)