There is a strange trend that if a smartphone is positioned as a 'mid-range' device, reviewers are willing to forgive it its many flaws. The camera is bad? No problem, it's just a mid-ranger. Snapdragon 630 (the same as in many 100€ Chinese phones)? Who needs anything better in a mid-ranger. The phone is made of plastic with major design flaws? Well, they had to compromise on something - it's a mid-ranger.
That would be understandable, if the year was 2016 and we were comparing Xiaomi Redmi Note 4 for 129€ against Google Pixel XL for 799€. But the year is 2018, and we get the reviews stating that Samsung Galaxy A6 for 350€ is a great mid-ranger, with compromises here and there, but for just 329€ it's a very good mid-ranger. 8/10.
And the same portals write reviews that Xiaomi Mi 8 for 299€ is not a great flagship. It may have all those features like the most innovative IR face unlock working in complete darkness, fantastic build quality, the fastest processor on the market etc But the camera is not quite as good as in Samsung Galaxy Note 9 (costing 1299€), and there is no wireless charging. So it's close to useless. Very poor flagship, 4/10.
Fortunately, not all mid-rangers are bad and overpriced, and not all flagships from the past year cost over 1000€. Most of the highly recommended mid-rangers can be bought for around 250€ and the same can be said about flagships from 2017. So which of those should you buy?
Unfortunately in this case it won't be possible to list advantages and disadvantages of going with an older flagship. Everything will depend on a model. There are some mid-rangers that are slightly faster than some 2017 flagships. And there are some 2017 flagships that have slightly better camera than some modern mid-rangers. Those differences are in most cases barely visible, and you would have to be really unlucky to buy a bad smartphone for 250€. Pretty much all of them will be at least decent.
If you know what is your main focus, you can easily find mid-rangers will be better than 2017 flagships in any particular aspect. However, going with a 2017 flagship is slightly safer option, as there is no risk on compromising on anything too much.
Take a look at few examples of 2017's the latest and the greatest phones, now being sold for around 250€:
Sharp Aquos S2 219€
https://www.ebay.de/itm/113235079228HTC U Ultra 239€
https://www.ebay.de/itm/272574470288ZTE Nubia Z11 Max 249€
https://www.topresellerstore.com/it/smartphone/1318-1435-nubia-z11-4-gb-ram-64-gb-rom-white-gold-internazionale.htmlHuawei P10 255€
https://www.ebay.de/p/234023712?iid=132762294200