If you want a really high-stability clock, you need an Oven-Controlled Crystal Oscillator, or "OCXO"
As the name suggests, the oscillator is inside a little oven, thus reducing the effects of ambient temperature changes.
These come in somewhat larger metal cans, but that doesn't mean that anything in a metal can is oven-controlled - it isn't.

The only advantage of an ordinary canned oscillator is that it saves you designing & building your own oscillator circuit - which you'd need to do if it was to feed more than just the processor.
Desgining a really good crystal oscillator can be something of a black art - which is why you might want to buy one ready-made in a tin can!

Of course, a cheap canned oscillator won't be as good as an expensive canned oscillator - and the same can be said for different "grades" of crystal. You just need to read the specs & pick something which meets your requirements.

If all you need is a crystal to connect to the processor's XTAL pins, there's no need for a canned oscillator.