Most players learn the two-handed backhand today, but the one-handed backhand will become more important as you get older.
Why? Because you’re going to lose speed, and you’ll have a longer reach with a one-handed backhand. There’s no better model for a one-handed backhand than Ken Rosewall’s smooth, easily repeated technique. He hit it almost exclusively with backspin (I prefer that term to “slice,” which means sidespin).
Another thing to learn from the Rosewall backhand: Though it was his best shot, he didn’t hit a lot of winners with it. He used it as a set-up shot. That’s a great way to think about your best stroke. Instead of going for winners, use it to make other shots easier.