Is plausible positive or negative?
Sentence two would be the better option. You are using the word plausible to mean "likely correct", "probable", or "reasonable". That your conclusions have a good chance of being correct is a positive thing, whereas the fact they are "inexact" is negative. To paraphrase, you seem to be saying: "These conclusions seem reasonable and have a good chance of being correct, but we cannot be exact in proving/defining them."