I have to admit that my opinion of Hifiman products is not the best... I don't like their upgraditis (i.e. too many models much too fast) or their sometimes really bad manufacturing quality (a flagship where the grates on the frame ruin the pads... a 1600,- model that's made of cheap plastic etc.) and I've yet to find a single sound signature I like: most sound simply bland to me, many have bad comfort (too heavy, too large cups that sit in the wrong places). I must also add that in general I don't like how planar drivers present bass, regardless of manufacturer.
So, in short, personally I'd never consider the Sundara, so I've never tried it and can therefore not compare it to the K371.
In general there's something called diminishing returns: ever so slightly better headphones tend to cost ever more if you're approaching high end... in my opinion you get very good headphones in the price range of 150-500,-, above that it may make sense to look at models up to ~1k, maybe 1,5k. And believe me, I tried...
So while many things are more a matter of taste than actual quality (which is hard to define clearly), there's no guarantee that something more expensive sounds better *to you*. And in the end thats all that matters (i.e. if *you* are happy with the product). And not what someone on the internet said
If you have the opportunity to test both in direct comparison, go with the one that doesn't kill your budget and sounds good -- many differences vanish when not heard in the context of a direct comparison anyway. Otherwise get the K317 and call it a day. It is a really good headphone after all.