I always prefer the train. Navigating to/within the train stations is a lot less hassle and aggravation than getting to/through/from the airports. The overnight train can save you a night of hotel cost (if you are one of those people who, like me, enjoys sleeping on a train). And it's just a more civilized way to travel, IMO.
I took the fast train for the first time last year and was extremely impressed. I've been taking the sleepers since the late 1980s and they haven't changed/improved much at all, but that fast train is a revelation in comfort and service. Mind you, I don't have a problem with the sleepers either, but they are definitely old-school.