1. 250GB SSD
2. "only for the OS and nothing else". This is a fools errand. Many/most applications you DO want on the SSD. Some must be there.
Trying to keep it 'only the OS' is tossing away 1/2 the functionality of an SSD.
3. The price difference highly favors a 250GB drive.
4. A 250 is generally a bit faster.