I'll see if I can give a good comparison of the two. I cannot give you an accurate description on how a stock 700 feels and rides, because I bought mine used with an exhaust and aftermarket shocks already on it. I never had a 400EX, but I have ridden my cousins plenty of times to get the hang of it. Power wise, they don't compare. The 700 is also much more comfortable ride around in the trails, especially when you are in a rough terrain. With the stage 1 elkas on my XX, it just soaks anything on the trail. It just glides over "whoops" on the rail road tracks. When you are on the 700 compared the a 400, you are going to notice how big and heavy it is. Because of how the suspension was designed, you cannot ride it like a regular sport quad. In stock form, you have to try harder. You have to put more effort into corners than a straight axle quad, because the 700 actually doesn't like to corner. But it will corner if you want it too. It has a tendency to carry the inside front tire up higher when cornering hard and fast, but it does pull around. That's what I miss about my Z400, is that I could whip it around with minimal effort whenever I wanted. I have to try a little harder on the XX. But I have more fun on the 700XX than I did on my Z400. I can ride it harder and faster in the trails without it beating on me. I love having the power. Since owning this, I have decided that I will never go down in power for my personal ATV. I have a lot of back roads to ride on, and I love going fast. 80mph comes pretty quick on it, and I never find myself trying to shift for another gear wanting to faster, as opposed to my Z400 when I was just breaking 70mph. Hill climbs are nothing for this machine. I don't bottom out in trails. Sometimes its hard to keep the front end down, all I want to do is pull wheelies. I race my cousins 400EX, and I can pass him doing a wheelie, and he's already going 65-70mph. I went down a tooth on the front sprocket and it will pull the front end up in 5th gear. Even without all the weight of the XX, I could barely get my Z400 to pull up in 4th gear. But, I have determined that the my 700XX is not a "putt around" quad. You gotta ride it hard and fast. The 400EX is more casual. The 400EX is like a basic quad. It is air cooled, doesn't need as much maintenance, not that exciting to ride. The rear end of the 700XX has a lot going on, but just like anything, you have to check on it. You have CV boots to check, and sway bar linkage to look out for. The chain though, is better protected. Because it is inside the sub frame, you don't worry about it as much. There is no flex on it, it doesn't move because it doesn't connect to a swing arm that is constantly moving up and down. Chain life will generally be longer, unless you really clutch the shyt out of it. My 700XX is 4 years old and still on the original chain and sprockets, although I swap out the fronts sprockets between the stock 14 and a 13 occasionally. I could go on, but this is all you really need to know about the XX.
700XX pros- Power for days, suspension, ground clearance, doesn't leave you tired and sore.
cons- Heavy, almost utility quad like handling, more maintenance.
2007 Honda Rincon
Twin Air Filter
High Lifter Springs
Viper Max 3500lb Winch
Warn Skid Plates
ODI Rogue Grips
Maxxis Bighorn 2.0
ITP Mudlite XL
2014 HD Dyna Switchback
Vance&Hines Pro Pipe/Tuner/Intake
2010 GMC Sierra SLE Crew Cab 4x4