DEA,
I had an '01 400SX before buying my '01 520 SX so I can give a decent comparison between the two. First off, the 400 is easy to ride with a good spread of power but it isn't like the japanese 400s or 450s. It's much mellower. You will want to gear it down substantially. For tight woods go down from a 14 to a 13 countershaft sprocket and up from a 50 to a 52 rear. This sounds like a lot but it really isn't. It will make 1st a granny gear and 2nd through 6th will be just right. Stock, the gearing is so tall the bike really doesn't have the motor to pull it. With this gear change and some jetting the bike is plenty powerfull and is versatile enough to do anything.
Now for the 520. :)
Man is that motor awesome! It's so powerful but easy to use. One ride on it and I knew that was the bike I wanted. Compared to my 400, it's just a whole lot more everywhere. It even runs smoother and will chug at low rpms better. The only real negative I can think of is the unbelievably tall 1st gear. Remember, this is a moto-x bike not an off-roader. Anyhow, 1st gear is taller than 2nd on my freinds 426!!!! I tried gearing it down but I can't find a combination I like. It's not as big a deal as it sounds because the motor will run at insanely low rpms. With all the flywheel mass, it's not nearly as stall prone as a japanese 4 stroke MX bike. (Unfortunately it's not as snappy either.) The only time the gearing is a problem is in the SUPER technical, trials type stuff.
As for chassy and suspension type stuff, I didn't notice much difference between the two. Some poeple prefer the handling of the 400 over the 520, but I didn't notice any differences. Even in the tight western PA trails we ride. Both bikes weigh the same and have the same components so the big differene is the motor and the price. Usually the 400SX's get overlooked and good deals are everywhere.
If have any specific questions, post back and I'll see if I can help you out.
Good Luck!