Do a search.
Pleanty of info on both models of tractors. They are both good but they are different size classes. You need to analyze what you want to use the tractor for and choose the best size that fits YOUR application. Obviously the 3540 is bigger, can lift more, and pull more. But the 3400 is easier on the lawn, more manuverable in the woods, and cheaper. You have to decide what is the most important.
That said, I bought the 3400. I mow my lawn with it, cut a lot of wood, tow trailers, bushhog, and a good bit of loader work. Yes I wish it were bigger for most of the jobs, but it gets them done, just a litter slower. But the reason I chose the size I did was because of price and the lawn mowing.