the fact that the charges they were facing were completely different factored in too. a lot of people seem to forget that paris only received probation for her dui - the 45 day jail sentence stemmed not from the dui, but from twice violating her probation. so frankly, nicole's 4 day sentence for her dui was harsher than what paris got for her dui. now if nicole violates her additional 3 years probation, she faces up to a year in jail, whereas paris' maximum sentence for violating her probation was 45 days.
yes, nicole probably would have gotten more time if she'd gone the paris route - pleading guilty, not showing up for court, arriving at sentencing 20 minutes late, blaming everything on her rep, etc. but the crimes they were on trial for when jail time was issued are completely different to begin with.