Would it be better to change the front sprocket or rear sprocket for more low end on my 400ex

 

Front sprocket would be more economical.

 

front sprocket will be cheaper, go down 1 tooth and see how you like it.

 

Go down one tooth in the front if you are into tight trail riding. Get a 13 tooth if you put 22" tires on the back. You can get one from sidewinder for sure.

You would have to go up 3 teeth in the rear to equal going down one tooth on the front. Rear sprockets cost more plus you have to deal with a clearance issue with your skidplate.