just bought an 01 yz250. great bike. motor revs quick and bike handles great. i have a friend w/a 98 yz400. also good bike. seems built for a larger rider. my 01 is a little small. the 400 is a little hard to start, but once started it rarely stalls. it handles good and has a power band that goes on forever. it does feel a little heavier than a 2stroke, but not in a bad way. personally, i prefer a 2stoke. but the 400 makes me wonder.....a little..
The only way to really know which is best for you, is to ride them. maybe find used ones and ask for a test ride....or better yet make friends who have different bikes. friends w/bikes are a good thing.