My top loader has damping springs at the bottom and a plastic "balance ring"[1] around the top of the basket that's filled with fluid to reduce vibrations. Certainly no concrete anywhere in it, that I could see when I took the cover/case off anyway. Though I've been having problems with it shaking too much during the spin cycle, so I wish it did have concrete. [1] http://www.appliancepartspros.com/whirlpool-balance-r…
Only front loaders have it. Top loaders don't spin fast enough to need it.
Front loaders have a harder time balancing the load due to the orientation of the drum, so require greater inertial mass to stabilise them.