Actually if the weather is marginal, your instructor will probably find a way to make the flight interesting anyway.
Strong winds make for good crosswind practice. Marginal visibility is interesting for a low-level navex. Low stratus cloud, assuming it's not above the icing level and the instructor has an IMC, is good for some instrument practice. Doing steep turns with no clearly defined horizon is much more challenging than doing them with a clear horizon. And so on.
So if the weather is marginal, make use of the opportunity to learn how to deal with it. You're going to encounter those conditions anyway later on. Better to have experienced them beforehand.
Although things like first solos should probably not be done in marginal weather.
As far as FSX is concerned, it's a game first and foremost. But it can be useful for things like flight planning, instrument flight, working with navaids, training circuit procedures, learning to work with checklists and so forth. It's not very good for visual navigation even with a local scenery pack, and the simulated R/T is distinctly American. It's also not very good for learning how to control an aircraft, particularly at the edge of the envelope (stalls!), unless you have a very sophisticated setup with force feedback yoke & pedals, and multiple screens. So be selective in what you're going to use it for, but then it can be a good learning aid.