A lot is going to depend on how you fish and how you intend to fish in the future. If your strictly a jig fisherman and fishing from front of boat a 45 in shaft is what I would recommend. If you ever intend to long line or pull cranks with it from back of boat then you need a longer shaft. I run a 52 in 55 lb thrust terrova on a stick steer about same size as your boat. Work great for spider rigging and pulling but I have to put motor all the way down when towards back of boat to keep prop from cavitate government at high speed. No comparability between a SI unit and terrova with Ipilot, you would have to step up to the link and larger unit to link. Where your fishing and how you fish would determine if added cost would be worth it. 9 inch would be better but perfectly happy with my 7 si mounted on trolling motor.
Proud Member of Team Geezer
Charlie Weaver USN/ENC 1965-1979