"I thnk comparing battery life with a Kindle is a red herring TBH."
To me it would just be the awkwardness of having to make sure that I plugged an iPad in each night where as a Kindle will just keep on trucking for weeks.
"Asl for flicker, I don't know. I'm fairly susceptible to eye strain induced headaches (got them a lot at work from computer screens) but, now I think about it, I don't seem to have that from my iPad. I've read for decent amounts of time on it with no issues but I am still very much a proper book fan so perhaps haven't tested it thoroughly."
Flicker is a part of it, the brightness is another, personally I don't find any computer screen as easy and comfortable to read as a proper book.
The iPad is pretty much a normal computer screen, so it will have those issues.
A Kindle is designed to be much closer to a book, so it's avoids a lot of those issues.
"But you're right, you can't compare the two as they are designed for different things."
To a certain extent you can, if all you're after is a good eBook reader, in which case the Kindle trumps the iPad. However the moment you begin to think of all the other things that an iPad can do and a Kindle can't even come close to you begin to see it's strengths.
If money were no object I'd want both as each can do something better than the other :)