It is your bike so its up to you. But you have got to take things into account. Its a 400ex It will never be like riding a Banshee. Do you have any work done to the Shee. Whats done to the 400? What kind of condition are both bikes in? Whats the years? Is the trade bike for bike? Does he have a title, do you? Are you sure his bike isnt stolen.
You said your bike doesnt start to easily. Do you maintain the bike. Start it up often and let it run. How often do you ride? I would never trade for a 400ex. But thats just me.