If time is the most important thing, I recommend the bus. Bus departures are very frequent. Journey time by bus is 75 mins, whereas the best train times are around 100 mins. (That would be for Keikyu line to Shinagawa, + N'Ex)

The added complication with trains is that the best route will depend on the time of departure. at 10.05am it makes sense to take Keikyu to Shinagawa and then pick up the N'Ex. But a few minutes later, you've missed that connection at Shinagawa and you're better off with another route.

Is there a chance that the bus will be delayed in traffic? Yes. But it's usually pretty punctual.

If the ability to work while travelling is more important, then I would prefer the train. Narita Express is an OK environment to work in (especially green cars) and even on the subway-style cars of the Keikyu line I can balance a laptop on my knees. By contrast, I can't stand working on the shaky bus.

Final note - For my money, the scenery that you see from the bus is mostly unattractive. There's no scenic route to Narita that I know of.