That depens on what you are planning to vary.
Process: Your device is probably produced on more than one wafer. No wafer is like the other. The production "process" will vary. This is captured by the process variation.
Mismatch: Now that device is existing more than once on that same wafer. But also here you have some kind of variation depending on where the device is located on the wafer. This is captured by the mismatch variation.
Now you can apply both or just one of them, realy depends on what you are looking for.
For a resistor the process variation can be around +/- 20% and the mismatch +/-5% ( depends on the foundry).
Process variation is usally larger than mismatch variation.