weekday mornings is probably the worst time to try and predict which route will be "the best" for your journey. The M1 has extensive roadworks Luton - Hemel Hempstead; the M40 is largely works-free at present but leads on to the jam-prone A40 and Westway. Both routes run at capacity, so the slightest problem (breakdown, accident, etc) causes huge jams to form very quickly. Expect your journey to take at least 3 hours, probably more, even though it's only 120 miles.
Don't forget you will be liable for the Congestion Charge, and some of the highest parking charges in the country.