Widowmaker, Wrangler, and Gunslinger is the best choice for a Highlander[footnote]Competetive TF2 gametype with one of each class, usually CP gametypes and maps[/footnote] game because the Widowmaker allows you to keep a supply of metal, assuming you can aim of course, and the Wrangler and MiniSentry allow for fast area denial and harassment of the other team at long range, since the thing's practically an aimbot. Normal sentry should only really be used if you're defending the last point and need more firepower.
In short, I like the Mini Sentry when it's used right, but most of the time it's not. If you have another engy on your team it's acceptable, but if you're the only one then use the wrench please.
In MvM it's pointless. Instabuild canteens and the instant build between rounds makes the increased build speed redundant. Setting up crossfires with two minisentries will just harass the bots and not do any sizeable amount of damage, whereas a level 3 with full rapid fire and health will kill most of the little buggers, and damage the big ones quite a bit. Disposable sentry is pointless in my opinion, and you should spend the money on a bigger clip size for your Frontier Justice, which may as well be mandatory simply because of the reliable source of crits.