You would want to do the right thing and be honest with the second company that you've accepted an offer from another one in which you'll start on Monday.
What are your values in terms with working for a company?
1) Long-term prospects 2) Health insurance 3) Autonomy 4) Team work 5) Working independently 6) More money 7) Company matched funds into retirement 8) Company culture 9) Team/division 10) Manager/Supervisor
You need to go with what you feel is the best fit for YOU.
