Upload & Sell: On
I shoot all the time from a kayak, and started with a 300 f/2.8 ii + both TCs. IQ and weight are terrific, but I really wanted longer reach, and was lucky enough to pick up a 500 f/4 ii about a month ago, and have now tried the 500 f/4 (with both TCs) extensively.
The 500 is amazing, and I love having the extra reach. Handholding the 500 is a little harder than the 300, but not so hard that I have ever wanted to reach for the 300. (In fact, I hate to part with the 300, but I suspect it will get very little use now).
Caveats: the 500 ii is about a pound and half lighter than the 500 i, but I think I could probably handle an extra pound and a half, and I'm a wimp. Plus, I recently had a great opportunity to shoot with Tim Kuhn, who handholds an 800 all the time. Also, one of the big reasons I went for the 500 was the chance to use the 2x TC (I have a 5d3 which can now AF at f/8).
Bottom line: If you think you might upgrade your body, and if you are in moderately decent shape (I'm a wimp), I would recommend the 500 i.