There's no black and white answer for this, a smaller person will want smaller increases. If you're training in a commercial gym, then 0.5 is probably too small and will get buried in the minute variations in actual plate weights. The short answer is, which ever will let you keep progressing the longest before you get stuck. Ideally, I think, each bench session should feel challenging, but not so challenging that you're constantly worried about missing reps. Personally, I did 2.5 lb jumps for a while, but I'm actually going to try using smaller jumps now, 1 to 1.5 lbs.