I dont consider Sidious cowardly for killing Plagueis in his sleep, I consider him efficent and intelligence. I see it as the same as him trying to flee a fight with Yoda. He has far too much self control to let his bloodlust effect his plans.
He has an agenda, a very wide reaching and long term goal and he isnt going to risk it in a single fight. He might calculate his odds of winning to be 60% or 70%. That means he has a significant advantage but it also means taking on a 30 or 40% chance of losing, and that isnt something that interests him. Part of what made him so successful was his cunning, He doesnt play the odds, he minimizes them as much as possible before doing anything. Once the fight with Yoda began Sidious was anything but scared, he loved it, but it was still a risk hed rather not take.
As far as Sidious vs Plagueis its hard to say. Plagueis was extremely powerful, and possibly could have almost equaled ROTS Sidious had he survived, but you have to remember the act of killing the master gives you a power boost. Plagueis had already recieved it and as of TPM Sidious hadnt so whatever advantage Plagueis held might not have been soley due to his natural power.
Add to that the fact Plagueis was free to spend all his time learning about the dark side while Sidious spent a lot of his time living the life of a politician. This gives Plagueis a big advantage as well since he had been able to intensively work toward reaching his potential, Sidious had been busy focusing on taking down the republic.
Ultimately I dont think Plagueis could have kept up. Maybe if he stayed the master until ROTS he could have stayed close, but once the republic was theirs and Sidious could spend some serious time learning more about the dark side he would have pulled away.