100 ppm means that the frequency will not vary by more than ± 100 parts in 1000000 or ±1 part in 10000 or ±0.01%.
If it's a 10MHz oscillator, 100 ppm means stability of ±0.01% or, in this case, ±1000 Hz.
All other things being equal, the ±50 ppm part would be better than the ±100 ppm, but you have to read the part data sheets to know exactly. The term "Stability" doesn't mean much.
Oscillators are usually (it's been a while, may not be exact) rated in ppm frequency change per degree C (or F), and in ppm change per year of operation, and in ppm per volt of supply voltage change. So you have to look at your requirements and see how that matches up against the detailed spec.
Crystals are just that, and you need to build an oscillator circuit around them. That circuit may contribute to the frequency errors, if not done correctly. Oscillators contain the circuit, the components needed to make it oscillate correctly, and an output buffer.
"decouple with capacitors"? depends on the type of oscillator module you buy and what you are connect it to. If the oscillator has a TTL output (0 to +5 volts) and you want to drive some TTL gates, then you don't need a cap.