Keep in mind that buses in Europe going from city to city, say Frankfurt to Basel, or so are much more comfortable than those in the US, if you are familiar with them. The last time I took a bus (internationally) was in 2009 going from Forbach in Lorraine to Saarbrücken, not much of a ride but comfortable and pleasant, and I could envision myself being on that bus for a journey 10 hours or more. Buses I have taken going from city to city have been in France, Germany, Poland, Sweden...all were satisfactory, comfortable.
The cheapest train fare would be to purchase the discount ticket going from Paris Est to Frankfurt Hbf, (49 Euro in 1st class) and another discount ticket Frankfurt to Munich on the ICE (maybe as low as 29 Euro in 2nd class), and a compartment seat on the Night Jet train Munich to Venice. Paying for a sleeper or couchette is obviously going to be more expensive...I take the Night Jet train sitting in a compartment seat. This day and night train option does require two changes but with more than ample time. You tailor the route and departure times...that's what I do. You can book the Night Jet now for a departure in Oct.
Now, if you prefer going direct if that can be done with the bus, then take the bus. In a way it is simpler, if you don't want to bother with changing trains, stations, etc. Convince the others in your party likewise. There are times I like breaking up the route but that depends, other times I prefer a straight shot, which you get on the night train.
If you have taken night trains in the past and found them disagreeable (noise, sleep, disturbance, whatever) to you, then fly, but that is another luxury, depending if you're to pay for it...I would not. It makes no difference to me whether I sleep on the night train or bus, I know I will, no problems.