i've tried bonsai repeatedly - goes okay for a while and then the plant
inevitably dies. it's a very precise thing...due to the manipulation of the roots, they need to be watered on schedule religiously. and for how much they cost (even the small ones) it's something to think about before investing. there are some really good sites out there - try googling and do some reading up on it.
i always thought orchids were hard to care for, so i never ventured. i took the plunge a few years ago - it'll be 5 years? maybe just 4, this christmas since i got my phalenopsis. fairly easy to care for - it hasn't bloomed since i got it though...i just recently changed it's location and i'm hoping it'll bloom at some point soon.
i recently ventured again and got a combo planter at bj's wholesale. a dendrobium with a bromeliad. i mist them twice a day and water weekly and so far so good.
orchids have come a long way over the years...these hybrids are a bit hardier than what was available a dozen years ago.
it's a pretty safe bet to purchase orchids these days - stores like walmart and target (where i got my phal) carry the types that are easily cared for.
i also got an orchid as a trade. tiny little thing - really pretty tho! one of the more tender varieties and i didn't mist it often enough and i lost it.
again, there are a lot of sites specifically for orchids...google and read up.
between the two, i'd recommend getting an orchid. unless you have the time to devote to bonsai (and someone who will follow instructions to a 't' if you go on vacation).