i have a greenspeed GTC
www.greenspeed.com.au
it's got the 16" wheels on it and nothing on that bike is cheap. has internal hub breaks and is VERY stout. also you'll find w/ the smaller wheels you get greater durability and strength, but you sacrifice some peddling speed and comfort due to the smaller sizing. however the thing is FAST on downhills, most DF's have a hard time keeping up. had a friend clock his trike at something like 65 MPH on some downhills. (i wasn't there so i can't vouch this is true but he's an experienced rider and i have a tendency to believe him.)
guess it all depends on the bike and the parts it's made of. maybe if ya post what your looking at we could help you a bit more on whether it's a durable peice of equipment. whatcha got in mind?
cheers
kat