When it comes to fiction I am not interested in participating in a work-in-progress, I have my own. So when I pick up a book of fiction I expect it to be finished. I do not expect to have an update come along next week with a new character/ending/whatever. And if the update is just fixing errors then either I missed them and so they didn't matter, or I saw them and it's too late to make me happy about them now.
Websites of supplementary material may sometimes be appropriate for fiction, but that is quite different from offering updates to the books themselves. And I certainly don't want to read a novel on a website.
Non-fiction is a very wide field and much harder to discuss, because any statement I may make about one subject is almost certain to be contradicted by another. But even in this instance I would say that an EBook is best used to represent some static/snapshot view of the information, presented in a manner suitable for offline reading.
The distinction for me is fairly simple: a website is a moving target, an ebook is a static one. And I much prefer that this distinction is maintained. In general, the usefulness of an ebook reduces in correspondence with how often it updates. If an ebook updates so often that it is no longer practical to discuss some particular edition, then all it has become is a backup of the website (which may have its uses but I would still consider it a backup/snapshot rather than an edition, whatever form it takes).
As for:
"I would favor an ebook 1) because of my general perception that books (including ebooks) are a little more "professional," and 2) I'd be able to read it even when I'm offline."
The first is dependent on who you are talking to, and which ebooks. On MR there is a widely held view that the majority of independently published books are rubbish - and this independently published author cannot argue that the view is not justified (I just don't think it applies to me!
).
The second I agree with to the extent that my Internet connection is tenuous and expensive; I would not want to be reliant on it for access to reading material. But there are various ways to save some website material for offline access if you want that.
But this little gem:
"The competition among ebooks, though stiff, isn't as hopeless."
made me want to weep.
Copied from Amazon earlier this evening:
New Releases
Last 30 days (189,777)
Last 90 days (558,875)
Coming Soon (51,176)
Four years ago the "Last 30 days" figure was around 86,000.
Sure the figures pale in comparison to the millions of new profiles being created on Facebook every month (a 2012 figure available
here, suggests it was then around 13 million in 30 days), but after a certain point the increasing magnitude means little to an individual's position within it.
Publishing is pretty hopeless. No sane person would do this unless they also enjoyed doing it for its own sake.
By all means, use both ebooks and websites, their respective advantages can complement each other. But I believe your best success will be in using the right tools for each part of the job. For lengthy, relatively static reading material, ebooks are great (epub or mobi for reflowable material, PDF for fixed layouts). For short and/or dynamic and/or interactive material, websites are usually a better choice; ebook technology just isn't up to dynamic or interactive use yet (or not with support across all common devices).