1st year 400ex. The swing arm has been beefed up it would crack, the frame at the swingarm connection has been beefed up also would crack and the clutch on the first year had a ton of problems, the clutch is still weak and you can smoke it by adding just more agressive rubber but the first year was realy weak.
The Z is really a EX w/ a more potent motor it does not loose power as the day goes on like an ex b/c of the liquid cooling but stock the 2 are very close only like 2 hp different, the thing is a few $ makes a dif on the Z the EX is pretty well optimized stock and bolt ons really don't add that much.
I won's sell my 02 EX to get a Z just b/c I have more power and better suspension as it sits so it would be stepping back but it would not take much to make the Z so much better. I can add a 440 kit to the EX and still be better than 90% of the quads out now.

 

Two of my buddies have z400 , one used to have a 400ex. He says he would go back to the 400ex if it had reverse. the z400 is alittle faster , mostly on top end.
In the woods and on a moto track, not much differance. I race all day with them and we stay together . the differance is so small , only a pro would notice it.
the z400 is a new quad, the 400ex is a proven machine. either way you wont be unhappy with your decision.
chow

 

just a note:

based on your kind of riding, you might not want to overlook the cooling systems. honda, as always, is air cooled. while the Z is liquid. if you do a lot of distance or spend a lot of time on the throttle then you might not want to have to stop to let it cool.

i have not owned either quad, so this is just general knowledge

 

I do not sell very many clutches for the 400ex. I sell alot for Yamaha. Frame braking can not say it does not happen but I have never seen one in my shop. As far as Suzuki goes they make a nice mailbox holder or lawn orniment after a couple years. How do you like the price on the Z400 parts. Front fenders $199.99, A-Arms $129.99 and so on. The price on the Z400 are double or triple the price of the 400ex. We have more junk Z400 around here than I can belive. The motor holds up but the rest of the bike bends and brakes. We have one here broke into. It hit a groundhog and went tumbling down the trail. The guys are trying to get factory parts for there Z400 but Suzuki does not have them and they set and you make payments on them. Lot of fun that is.

 

I had a 1st year EX and it I wated on parts as well Honda at the time was making all the bikes they could. Same for suz, 1st year you do not know which parts you need so you do not have stock of them plus you are making tons of bikes so your production lines are going 100% w/ little room for spare prats. It is all part of the first year bike thing.
As for clutches not many folks go back w/ stock. Also how many EX stering stems have you sold? Order one and see if you do not wate at least a month for an after market unit.

 

Steering steem three days at $105.00. Honda,Polaris and Yamaha no parts problems. Suzuki and Kaw. Week to 6 Mo.. I will not even sell stock Suzuki and Kaw parts anymore because of that problem. If you are thinking about the new Suzuki call a dealer and ask for a A-arm. front fender and steering steem check the price and when you can get it. Then call Honda And ask for the same parts for the 400ex and check the same.

 

I can get a stock stem for honda but I bent 2 from landing big doubles the I bought a third while waiting on an after marke unit and now it is back up.
I said b/f I would not trade my EX for a LT just b/c all I have in it but if I were going to buy new I would go LT yes I know parts are slow but I would never replace a-arms of stem w/ a stock piecs also I try to never buy 1st year bikes the EX had very slow parts the first year. The thing is stock for stock the EX is close enough to the LT that it is up to the rider but the potential of the LT is far greater. Now in a few days Honda may answer w/ a hotter unit.
I have been the guy who defends the EX against a bunch of crap slung at it but I also know that the potential for the bike is near its limit the air cooling sees to that, I can watch my lap times go down as the day gets hotter and I see what how much harder I have to ride to keep clearing longer jumps. The EX is a great bike the LT as sits though is just a step up.