It doesn't matter as long as you have equivalent rinsing of the grains. See How to Brew - By John Palmer - Tun Geometry and Flow Potential
All things being equal, I suppose theoretically perforated would be better, since you can get more holes per sq in than slots. But I really don't think it matters.
Beer, happy Produce of our Isle/Can sinewy Strength impart,
And wearied with Fatigue and Toil/Can cheer each manly Heart.
Labour and Art upheld by Thee/Successfully advance,
We quaff Thy balmy Juice with Glee/And Water leave to France.
Genius of Health, thy grateful Taste/Rivals the Cup of Jove,
And warms each English generous Breast/With Liberty and Love!
(Rev James Townley, 1751)