Don't have a Grizzly, but a SeaArk - but wanted to share this:
If you are looking at a 20' now, go ahead and get it :D
When I bought mine, I was looking at 18 and 20, ended up getting
the 18 due to "sticker shock" but have regretted not getting the
big one ever since. Its not so much the two foot of length, or even the
extra width, as much as the higher sides. Mine is 21" sides, wish I had
30" when the river is bad.