I think the biggest difference is they do a better job agitating the
clothes. In a top loader, the agitator tries to swish the clothes
around in the tub. In the front loader, the clothes are constantly
falling on top of each other. IMO, that does a better job of
circulating them all around constantly and randomly. In my Kenmore
toploader, on the normal speed setting, I can see clothes that stay in
the same position for a long time, like on the outside of the tub, not
moving much at all.
How much of a difference this makes in how clean things come out or
whether it's worth the big price difference is another story. As to
using less water, while it's true that the cost of water isn't that
much in most places, the cost of hot water is definitely a factor.
Again, how much you wash will determine how much that amounts to.