First off West TN, welcome to the board! We are glad to have you here.
Man, your question is tough. As someone who owns a Supreme, it would be tough for me to recommend anything else.

I can see the benefits of the Gheenoe for certain rivers (or a lot of rivers) and that choice is perfectly fine and one I might have made had I not fished from a Supreme first. I believe they are fine craft and David (Woodsman) can attest to this (BTW, he has one for sale).
From what I understand the quality of the Riverhawk is questionable. (i.e. quality control issues) I don't have first hand experience but do know of those who have and they might weigh in.
I think you should do your research, talk to as many people as possible, read reviews, yada, yada, yada and make your decision from there. As well, some factors I weighed were resale value of the Supreme (will hold its value better) and all day comfort and all around usability as boat for both lakes and rivers (if you want it for more than just river fishing).
Feel free to ask any questions you like!
YMMV