JR or Keisei Espress?

I am departing Tokyo(Nihobashi) on Jan 4th 6pm(flight departure time). Is Keisei Narita Express faster than JR to Narita? Since most people are back to work, it must be rush hour traffic. How much time should I give myself if I want to be in Narita by 5pm? Thanks.

If you depart at 6pm, I recommend to be at the airport by 4pm at the lastest.

Is Keisei Narita Express faster than JR to Narita?

The fastest and cheapest way would be by Asakusa Subway Line from Nihonbashi to Aoto and then transfer to the Keisei Limited Express to the airport.

Taking the Narita Express via Tokyo Station, the Keisei Skyliner via Ueno Station or a limousine bus would be other comfortable options:
http://www.japan-guide.com/e/e2027.html

Since most people are back to work, it must be rush hour traffic.

No. Rush hours do not start before 5pm.

the offical latest check in at Narita airport is 60 min. This means you must have finished your check in and must be holding your boarding pass not later than 61 min prior to scheduled departure time. So if you have no priority check in or electronic pre-check without luggage an arrival at Narita 60 min prior to flight time is too late.

John Zee, a slight correction,
The "Narita Express" is a JR train.

Keisei operates the "Skyliner" and "Limited Express"

Yes, I will get 2 hrs ahead. I think of taking the JR Nex train, because when I buy the NEX/Suica pass, they sell the NEX reserve ticket for $1000! However, just wondering if I can share the Suica pass with my son? Because we will not know however subway rides will we actually take? Thanks again for all the replies.

No, Suica cards cannot be shared:
http://www.japan-guide.com/e/e2359_002.html

And NEX portion of a NEX/Suica package can be used only for coming into Tokyo, not when you go back to Narita.