More maintenance than plants, but much more fun too. Actually, depending on what type of plants you're talking about it could be more to maintain them than the pond.
All you really have to do is make sure the water stays clean and the pump keeps working, skim the top every day for a few minutes and keep the fish fed. You can get conditioner for the water that is safe for the fish. It's basically like having a huge fish tank, except that koi will survive in a wide range of environments so the care you put into the fish may not even be very much at all.
You can also landscape around the pond using a simple drip system. This is probably what you'll want to do anyways since the pond will look pretty bland without some color around it. It's really not that hard to maintain any of this though, really. I say go for it. Look up some basic designs on the Internet and go from there.