My dad, getting a telephoto

[RebelXT-WH2CYK]

My dad has a 20D and a 50mm 1.4L that hes been using for a while, and he is looking at the following two telephoto lenses for our trip to bermuda this summer.

http://www.bhphotovideo.com/bnh/cont...ughType=search
http://www.bhphotovideo.com/bnh/cont...ughType=search

We will be taking a cruise ship and he will be using the telephoto from the ship. His question is, should he get the 300mm f/4L IS or the 400mm f/5.6L non IS lens? We've never taken a cruise to bermuda before so we're not sure what the coast line will look like in terms of distance from the boat.

I figure he should get the 400mm, but the IS in the 300mm is tempting too.. with the 400, I could stitch the photos together to make a "more zoomed out shot", maybe something like a panoramic photo, but we're not sure about the zoom difference between the lenses...

How much of a difference does the IS make? What lens takes clearer photos? Has anyone ever been on a cruise to bermuda before?

thanks for taking the time to read this.. ;-)

***Edit**

How would a 1.4x teleconverter + 300mm IS f/4 be? it gives the option of 300mm f/4 or 420mm f/5.6, would it still be as sharp as the 400mm by itself?

so confused!