I think the best reason for going with either OS 4.0 or MorphOS would be how well it runs OS 3.x software. From what I've gathered, MorphOS does it quite well. It will be a tough act for OS 4.0 to follow, but we'll see how well it preforms when its finished.
My advice to a buyer would be to get whats available now.. but having said that, I would prefer to go with the official direction. So I've purchased an AmigaOneXEG3, which I should recieve by the end of next week.

But thats my honest opinion.