OP- everyone is right. Miniature horses and shetland ponies are very different from eachother, and really can't be compared at all.
Being a pony, shetlands are definitely on the trickier side, and generally need a very stubborn handler who is patient and willing to do a little bit of shoving around to get their way. I've honestly been thrown by more shetlands than I have any other breed, including thoroughbreds and arabians ;) they're trickly little buggers who know they're cute and are extremely smart. HOWEVER. With the correct training and a good handler, they can be very nice little animals. We have two shetland ponies that we use for therapeudic riding for children, and they are wonderful with the kids....if they have a confident handler. Those same two ponies will throw a tantrum and try to run off on you / shed you on a fence the second you put a bit and a half way experienced rider on their back. They can be good, reliable animals though, like I said. Our little gelding has loads of stamina and is the one pony I can rely on to work until he drops...as long as I don't dare treat him as any less than the royalty that he swears he is. (he comes from a long line of fuzzy wuzzies you know!)
As for miniature horses...they should have the same personality as their larger 'cousins.' Most of their 'tude and crappy manners are simply because they're treated like puppies, not full sized horses. However...I have met a few minis who acted suspiciously pony-like...my miniature mare in particular. I swear that she's part donkey, part pony- not purebred miniature horse ._.