Depends on your day one launch.
If you get enough sales on day 1 to get a half decent or better rank, then put it into KU.
If it's just being tossed in the abyss, then go wide.
KU is not a rank booster on it's own. Day 1 is all about sales and your mailing list, especially from pre-orders, and the rank you get from them boosts KU.
So after some rank hits, (or in my case I get downloads from my mail out), the book is visible in KU, and you get downloads which help the rank, and you start seeing reads late in the day and on day 2.
If you don't have KU readers on your mailing list, you won't see any KU activity until the sales rank kicks in.
Sci-Fi does very well in KU, but nothing does well without a launch.