The Rocket is basically a minimally updated 33cc Stingray, and the Shark is a 51 cc unit. Bigger engine, heavier. I had the Stingray for years and absolutely loved it. Also used my buddy's Shark a lot. That went thru anything. No probs going thru 24" ice with the Stingray/Rocket though. Other than that, not really any differences.
If you''ll be doing thicker ice, and a lot of it , and weight is not an issue, you might want the Shark...
but whatever way ya go, I don't think you can go wrong, IMO.