Buying a lightly used atv for my 10 year old. He is tall for his age and has been riding quads since was 3. Of the 2 mentioned which is better. He has a Dvx 90 now but is getting too big for it. Or are there others I should consider. Automatic is a must so no Honda or yamaha

 

I had bought a ds250 for my wife back in 2006. They had just come out then. She liked it. Actually bought it for her to ride while I worked on her Polaris TrailBlazer 250. She said it was easier to steer than the Polaris. I didn't really notice. It definitely lacked the zip of the Polaris, but we're comparing a two and four stroke. Now it was narrower, so it did like to tip a little more. Hopefully they have more aftermarket stuff for them now. It was pretty much non existent when they first came out.
Not sure about the 300, but if it's wider than the 250, I'd take it for that alone.

Most of all, I'd go with what he feels the most comfortable just sitting on while it's not moving.