Are border checks on the Romania/Moldova border tougher when crossing by bus than by train?

cdm has already noted his experience with the bus. Let me add to this by stating that, in my experience, the border checks by train, both ways, were just as perfunctory. The only thing unusual was that, when entering Moldova, a physician was present to check for any health-related problems. This check was limited to a brief visual examination and asking me whether I was sick. He was satisfied with an answer of "no". The entire "health check" took about 30 seconds. If you're not obviously suffering from a virulent disease, I think you will have no problems.

That said, it should be noted that crossing the border by train is a very time-consuming affair because of the change of gauge. The border guards collect everyone's passports before the process begins, take them away for processing, and then return them a couple hours later once the train is ready to depart again. In theory this gives the guards plenty of time to conduct very detailed checks. However, the face-to-face interaction I had with the guards was extremely brief.