it's really tough to compare across eras, I'd say they're about at the same level

Serena's serve is probably better, and her backhand is definitely better, but Steffi's forehand was a transcendent shot that nobody could really handle. I would bet on Steffi on clay, on Serena on hard court, and it'd be 50/50 at Wimbledon (with grass being the way it is now...on 80s/early 90s grass, Serena would likely dominate her thanks to her serve).

Serena is definitely top 3, maybe GOAT (it's between her, Navratilova and Graf)