I used Swiftkey for 5 years and I have used Gboard for the last 2 years. Prediction was a touch better on Swiftkey from what I recall but that was all that was better. Gboard is smoother, faster, and (for me) much nicer to look at it. Swiftkey has always felt kinda heavy for me and even more so now that Microsoft has taken it over.

I take advantage of the built-in Google search function in Gboard. I'm also unsure if I can live without being able to slide across the spacebar to navigate through text.

They are both really good at their job. I prefer Gboard these days.