I have a few small gripes with the way the film is structured, but that might be rectified by repeat viewings, and it's not really that big of an issue. Also a lot of the dialogue is hard to understand due to all the noise from the war, but what they're saying pretty much doesn't matter at all for the most part, so it's also not a significant problem.
70mm Imax looks pretty damn great. Definitely worthwhile. I'd say if you're forced to choose between the two go with the Imax over the 70mm. The vast majority of the film uses the full screen and it only shrinks down to regular widescreen in a handful of spots. So if you see it in a regular movie theatre most of the shots will be cropped. but if you can see it with both 70mm and Imax then definitely go for that.