Be aware of a major downside to upgrading! It's not just about performance but about changes in the interface, especially on key apps.
Safari under iOS 8 is a full screen app. What that means is that it operates unlike the desktop Safari or like Safari under iOS 7!!
The bookmarks and tabs are hidden, a frustrating design that means it requires two steps whereas it always used to require only 1 to move between tabs! You can turn back on the bookmarks (IIRC-- I'm away from my iOS 8 Mini RD), but there is NO option to keep tabs displayed. It also means a visually busy, distracting screen as the tab bar hides,and reappears depending upon how you scroll! Distraction City!
Also, while one may be tempted by dreams of 3rd party keyboards, count us among those who find them plagued by issues- see reviews on iTunes. So, that's not really a benefit.
The one big plus, the panorama mode, is a treat on the Mini, but photography is ungainly on the Air, so it's no big gain.
Others, of course, enjoy these features or aren't bothered by the changes, but the problem is, once you take the plunge, you can't go back! If 7.1.2 is running fine for you--and there's no app you absolutely need that only can run under iOS 8--stick with 7!
<Edited by Host>